Mike, welcome to the forums!
In order to answer your questions, we need more information. Are the propane tanks the only thing missing? If so, you can get a pair of 30 pound steel tanks from any RV dealer, probably for less than $100, empty.
Is the toilet the only thing missing, or are the fittings and mounts and holding tank missing as well?
As far as what to look for, floor rot, leaks, and worn out axles are the biggest problems. The air conditioner and fridge are big ticket replacement items as well, if they don't work.