I helped a friend convert one of the old 'Yellow Demons' several years back. After looking it over and considering all that had to be done and the amount of labor & materials involved, I advised him not to do it. But he decided to do it anyway.
I would not do it again.
Part of this may be solved with new buses - I don't know.
First big problem was no insulation & too many leaky windows (air & some water). Pulling off interior panels to add insulation, having metal cut to cover some of the window openings. Two roof A/C's, welding a compartment to add a generator, plumbing, wiring, holding tanks, then you had to build cabinets & furniture, sinks, toilet. Then repainting the exterior. By the time we were done, he could have bought a used 12 to 14 year old class A, for what he spent in converting it.
However; if the bus you are looking at is in good shape and one of the bigger diesel rigs already with a generator & A/C's, it might be worth it.