Welcome. I don’t want to sound crass but it seems like this is a ready, shoot, aim strategy. You say you bought 2 units only to find you couldn’t use them and now another that needs rehabbed but no experience, know how or plan?
There are lots of resources for parts materials etc on line and lots of how to stuff on online as well. This forum is a good place but you might want to be very specific about particularly what you are doing and break it down into individual items. Most dealerships are happy to take your money to work on your rig. CW can be a little stuffy when it comes to that. Find a nice local dealer with good reviews. LP can be filled most anywhere, campground, dealer, truck stops etc. Dumping is pretty much the same, campgrounds even if you don’t stay there might charge you a few bucks to dump, so will a dealer. Some truck stops have dump stations too. AC and refer serviced, those are dealer items. Prepare to open your wallet. Especially if you are not sure what’s wrong.