We bought our camper used also. The previous owner winterized it, so we ASSUMED both tanks were pretty empty and clean. NOT SO...ughh. I decided to open the grey tank and what a mess! There was all kinds of sludge coming out of there! Well, I got smart and put a 5 gallon bucket under the pipe before I opened it and YEP...FULL OF POOOOOO!!
I had to haul that into my house to dump it in our toilet, scrub that bucket with bleach and wait for the local dump station to open so I can empty that $#it out! In the meantime, I filled the grey tank by passing a hose through one of my compartments and running it into one of the sinks (faster than just opening a faucet). Added a cup of dawn dish liquid and let it sit for a day. Emptied it again, filled, rinsed, filled rinsed. Then I added some bleach to my holding tank filled it up and opened up all the faucets to sanitize the lines. (because anyone that leaves their poo in their tanks over the winter can't trusted to clean their lines either). After that was done, I waited patiently to drive the camper to the dump station. We dumped both tanks, filled both tanks numerous times, and dumped over and over. I have one of those tank cleaner hoses that goes down the toilet and spins to reach corners and sensors of your tank, so I used it in the grey tank first, then filled it and dumped it. Did that one more time and it was running pretty clean. Same thing with the black tank. It took probably 6 fills and dumps with two cleaning cycles. It wasn't perfectly clean, but it was the best we could do untill we could drive it around with some cleaner in it. We bought some stuff called Extreme cleaner and put it in the tank with a couple gallons of warm water in both tanks. By the time we got to our first campground outing, I was ready to dump! We also purchased a clear elbow for the exit pipe so we could see if the water was running clean. After a few of these clean/drive/dump cycles the tank water runs clear and our sensors read beautifully. Good luck! I'm thinking the red stuff is sludge from the sinks, etc. mixed with antifreeze (it's pink) and you'll need to empty it out, put some cleaner and water in, drive around to aggitate it a bit and dump again. It may take a few times like it took us to get them clean. I feel your pain...
2014 Rockwood Roo 23IKSS
2012 Dodge Ram 1500 Hemi 4X4
Me, Hubby and Luna the Lab