Craigslist is generally my go-to resource, but if you can’t find something rust free in your area, I would try to purchase something from outside of the rust belt.
Very few cats will adjust to or enjoy camping. If yours doesn’t, I would strongly recommend a pet sitter over torture.
Older camper vans in excellent shape can often be found for under $2500, and be ready to go. The older stuff is the only stuff that makes good financial sense to me, and I have also found them to be MUCH more reliable than the newer computerized stuff. I’m driving a 1973 high top Dodge camper van with 450k miles on it and it still runs like new. In the last year it has traveled from Florida to Minnesota, then across to Washington state before heading back to Florida via California. Just make sure you get is checked out by a competent mechanic before or after purchase and get everything it needs fixed before hitting the road. Pro-active preventative maintenance is always a good thing on any vehicle.
"Be the reason someone smiles today!" ~ Van_Dweller