If you only spend a few thousand dollars to buy it and realize that it is going to cost you much more to make it safe to drive and comfortable as possible to live in, you can try to peddle it for as much as you paid for it before you spend a lot more on it and feel "stuck" with it. Don't get caught up in "as long as you are going to fix this, you might as well fix that". RV repair labor rates are $75 per hour plus parts which aren't cheap. Pulling a generator out of the RV to diagnose a probem costs $300+, repairs often total over $800.
replacing a roof AC unit can cost over $1000. Furnace repairs are expensive too. People who have tools, work space, time, and truck mechanic and all-around handyman skills, including RV electrical and plumbing and AC/propane appliance repair can do well buying and getting an old RV ready to use safely and save about 1/2 the money. Learning while doing can be expensive and time consuming.