I think I saw that in a movie once.. . .
I think the main issue with converting a city bus would be that they have a flat floor and low roof (14" floor, 10'6" to AC). There's not a lot of room in it to add plumbing, tanks, batteries, etc. Additionally as the engine is on the 'trailer' part I'd assume it would be considered a 60ft vehicle vs a 40+20 and therefor too long for most roads and licenses.
I think something like a Vanhool Astromega
double decker would be a better option.