The TC camper in my signature is just about perfect for what you want. I looked long and hard and bought it for 5k. They were asking right around 9k at the time IIRC. It took them about 3 or 4 days to accept my offer. It was almost in perfect condition and always stored inside. Has 45 gallons fresh water and the same in holding tanks. At the time the people sold just the TC. Then about a year later they called to see if I wanted to buy the ford F350 dually for 15k. The truck they started higher but came down quickly. It only had 70k on it was a 2002 I believe with the prized 7.3 diesel. I almost bought the truck even though I didn't need it. But it was only a 2 wheel drive. I would have driven that truck for 100k without even thinking about it. So it can be done. My TC has a dry bathroom with a shower. Not sure why you would want a cassette toilet.
My point is you can find what you want but you will have to look and be patient.
But I think you should look for a 1 ton truck for full timing. JMHO
