This isn't hard, and many have given good ideas.
1. Use an RV pump...already recommended several times. They are 12 volts, and they can move water from a jug to your freshwater gravity fill easily. Add enough wire to reach from the pump to alligator clips to the tow vehicle battery or the RV batteries. Run the TV engine when pumping so you don't drain your battery. Use some simple 1/2" braided line on the input and your city water hose on the output. You will likely need to adapt from the fitting on the pump to a male garden hose fitting.
THE PROBLEM WITH THIS METHOD is keeping everything clean.
2. The hot setup. Go all in. This may be my plan.
50 to 65 gallon water tank to fit in the bed of your TV. https://www.plastic-mart.com/product...tank-fs0065-24
120 volt water transfer pump. https://www.northerntool.com/shop/to...hoCnkgQAvD_BwE
A sheet of plywood to mount the tank and pump so you can slide them out of the bed between trips.
ONE BEAUTY OF THIS SETUP is that the connection between the water tank and pump can be permanent, and the water is drawn from the bottom of the water tank. No priming.
3. What I'm currently doing:
Four Reliance 7 gallon jugs: https://smile.amazon.com/Reliance-Pr...s%2C227&sr=8-1
A couple spare caps: https://smile.amazon.com/Reliance-Pr...156HFVDM03ZEWK
Make an adapter using a 1/2" NPT to 1/2" barb plastic fitting: https://smile.amazon.com/Spears-Fitt...2362856&sr=8-4
About 15 to 18" of clear plastic hose and a hose clamp.
I plug the hose end into the fresh tank fill, then lift the jug and pour. No ladder, no funnel nonsense.
Each jug weighs 60 pounds full.
I carry 28 gallons in the TV, and it takes about 3 minutes to dump the jug into the tank.
No pumps necessary, and I don't lose water.
I use a giant ziploc bag to store my adapter and keep it clean.
If the lift is too high, get a stool: https://smile.amazon.com/Camco-Adjus...2363024&sr=8-1
This is also awesome as a first step when you're on a slope.
As for using your winterization port to move water, it can be done. But in most cases it's about as convenient as heart surgery. My pump and equipment are under the dinette seat, behind a screwed down piece of plywood. Once a year is fine. One or twice a camping trip ain't happenin'!