My tactics - FWIW.
Start clean. Use Clorox Cleanup spray or similar to sanitize the end of your garden hose(s) at every fitting exposed to routine use, on a paper towel to clean around the fill pipe, and to clean the holding tank fill adapter. https://www.amazon.com/Camco-40003-W.../dp/B0006IX850
I spray it on and let it work for about 10 to 15 minutes, and then I flush out the hose and rinse the fill adapter before filling my holding tank.
Keep the garden hose end clean at all times. As I'm using it, I hang it on the hand-rail at the entry...I don't put it on the ground once it's clean. If I'm filling jugs of extra water in the bed of my truck, I use the tailgate supports to capture and hold the hose off the ground.
I'd never fill at a gas station or other supply without sanitizing the fittings and the hose. I use this sanitizing technique when connecting to a campground hose fitting, too. If a dog peed on the hose fitting, I don't have to worry.
Next...Sanitizing your camper's water supply.
Sanitize the "shore" hookup (for RV parks that supply water). You can use a new hose-end fertilizer sprayer (e.g. from Miracle Grow) that is first washed, then add a very strong mix of bleach and water...say 50/50 to the fill cup. You need one that has both male and female hose connectors with a removable spray nozzle...I found a Miracle Grow (or similar) model at Home Depot. Insert the hose end sprayer containing the bleach concentration into your hose line BEFORE your sanitary connection hose...after your garden hose. Water flowing through the sprayer will atomize the bleach and add it to the water in much lower concentration to sanitize your hose, trailer fittings, and all the supply plumbing in the trailer. Open all faucets, showers, and flush the toilet several times. Be sure your hot water heater is in this loop so it's sanitized, too. If you can't find a suitable model of hose-end sprayer at a home store, this will do the job...simply put the pickup line in the jug of bleach. Hozon Siphon Mixer - Chemical Applicators | amleo.com
Next sanitize your holding tank and re-do the rest of the plumbing by using the pump... If you already cleaned the hot water tank with the first step, you could bypass it for the following steps. Pull the Anode/Plug and let it drain while you do the rest.
To first get rid of RV antifreeze and anything else in the tank and/or pump, open the drain petcock on the fresh water tank, and then begin adding water via the fill pipe using your hose. The tank will be draining/flushing as you're filling. But your garden hose will fill much more quickly than the tank can drain. Simultaneously run substantial amounts of water through every faucet, shower and the toilet using the pump. Any residue remaining in holding tank and transferred through the the lines will be flushed out.
Stop the pump, close the faucets, etc., turn off the hose, and allow the holding tank to drain through the drain petcock.
That should get rid of any residue, but your tank and pump are not sanitized yet.
Close the holding tank drain petcock, and sanitize the tank by adding the appropriate amount of bleach and refilling the holding tank. Let it sit for several hours to allow the chlorine to kill any bacteria. Then use the pump to send bleach/water through all plumbing via faucets and shower and toilet flush. Re-doing the lines and faucets with this bleach/water will remove any contamination introduced out of the holding tank during the purge. Flushing the toilet merely introduces sanitary water into that leg of the supply plumbing to ensure that no contaminants remain anywhere in the cold water supply line.
Once you have sanitized, the whole system will stink and taste of chlorine. Flush all systems...both from the holding tank through the pump and from the "shore" supply hose with fresh water. Be sure your hot water heater is back in the loop (with the Anode/Plug installed). Use lots of fresh water to flush out the chlorine or you'll be living with it for several trips before it finally dissipates. It can be surprisingly strong...far more than a city water supply. If you can afford the water use, fill and drain the holding tank several times while pumping water with the pump and with the drain petcock open.
Your water supply...from both a garden hose or the holding tank...should now be sanitary and relatively free of chlorine odor and taste.
Right or wrong, I've only sanitized my system once...when I bought the camper. Rather than introduce "contaminants", I simply keep it clean...and dry in the winter.
I do NOT use RV antifreeze in my fresh-water system. Instead, I have a serious air compressor that I use to blow out the lines. The key is to set the output pressure to about 50 pounds to ensure that you don't blow up your plumbing. 50 pounds is about standard pressure from a garden hose, and that's the pressure passed by a Camco pressure regulator. This compressor will blow the lines completely dry. Feed the air into the "shore" supply hose fitting, and simply go from faucet to faucet to shower to toilet about 3 or 4 times each until each blows dry.
My HW PUP has an outdoor kitchen and the plumbing to the indoor kitchen sink takes a convoluted path around the street side, rear, and curb side of the camper. It still blows dry. Inside a cabinet near the outdoor kitchen, there are also low-point drain valves in the PEX, so I open those repeatedly, too.
To fully drain the fresh tank, make sure the camper is level. If you shine a flashlight through the plastic holding tank, you can see if there is water in there.
With the fresh tank empty, I also turn on the pump briefly and let it suck air to be sure no water is caught in the pump, and I leave the drain petcock on the fresh tank open for a few days to give it time to fully dry...then I close it for the winter to prevent vermin from entering through the 1/4-turn ball-valve petcock.
Once I've drained the hot water heater, I can use a blow gun on my compressor to flush out most of the water remaining in the bottom of the tank. I leave the Anode/Plug out and the pressure relief valve open for a few days to promote drying, then I replace the plug and close the pressure valve to prevent bugs and critters from getting in.
I've use air to dry the water lines for more than 7 years on two consecutive PUPS, and I've never had a freeze. Until this March, I actually used a small pancake compressor to do the job, but I had to let it fill many times to get the job done. My new compressor delivers so much air that it does the whole job without ever having to restart.
p.s. I use RV antifreeze in the drains...especially the J-traps...and in the black and gray holding tanks. Those don't need to be sanitized or cleaned for "taste."
Compressor: Husky 30 Gal. 175 psi High Performance Quiet Portable Electric Air Compressor-C303H - The Home Depot
Pressure Regulator: https://www.etrailer.com/RV-Plumbing...BoCjQEQAvD_BwE
Air to Garden Hose Adapter (for connecting the compressor to a garden hose fitting): https://www.amazon.com/Camco-36143-B...FE1THWYDP25HGC
It took longer to write this than it does to winterize my PUP with an air compressor. Sanitizing is another matter, but, as I said, I sanitize when new and just keep it clean. I haven't had any problems with contaminated water.
Again...for what it's worth....FWIW. Others may disagree, but to each his own.