roadtoad, you lost me. Sikaflex 252 is a polyurethane adhesive, very much like a caulk and I can see using it on leaky seams, etc, before spraying the polyurethane.....but if you are going to "wiped down with a good automotive wax and grease remover" you should do that before applying the Sikaflex or anything else.
I would also phosphate (quick & easy) any rusted places during the surface prep. The elastomeric roof coating should be put on top of the polyurethane foam, not under it (unless you are saying to skip the foam).
Maybe I misunderstood, but this bus is going to be parked in an RV park. isn't it? Spraying polyurethane foam (2 - 1" layers, painted over with 2 coats of a white elastomeric rolled/sprayed 90 degree to each other) directly to all kinds of roof surfaces with normal paint prep is a very common roofing practice, especially for resealing metal mobile home roofs. The coating needs to be renewed periodicaly. Polyurethane caulk is what is used for minor repairs to polyurethane foam and is some incredibly sticky stuff.
bartthebus....I think you have a great idea, although a pain to implement due to all of the mechanical gear under the bus that would need to be masked off. Reparing/replacing anything (plumbing, electrical, bolts) sprayed over would be incredibly difficult and you would have to maintain clearances from anything hot or moving. Also the sprayer needs to be held a certain distance from the surface being sprayed. The guy spraying would need a flat surface to roll around on and several helpers to manage the hoses and facilitate his movement. Finding a company willing to do this might be a problem. BTW, spraying polyurethane foam is not cheap, and it doesn't come off without a fight. Been there, done that, roofed for far too long.
One more thing just occured to me, if you seal the bottom you better not have any major plumbing dissastors inside because it won't drain out.