It depends on what he intends to do with it. If he intends to park it, it is likely that tanks are not needed. If he intends to tow it around, I doubt its overall construction will permit too much of that. They are built primarily to be towed to a site and left. They can be moved from site to site occasionally but....
It is possible that it also has household type appliances like 120V refrigerator, stove, toilet and perhaps has little or no 12V wiring. If tanks are to be added, a lot of plumbing changes will also be needed and the appliances, toilet changed as well as new tires. Underneath the chassis would have to be looked at to see if its construction will permit this addition (location and depth).
If towing is the goal, he would be better off re-selling it and getting a 'real' tow behind trailer.
May have been a costly lesson!!! I hope not!!