With an older model mobil home what you see is what your going to get. There really is no way to add insulation, with a vaulted ceiling the heats going to rise and be useless because it's trapped up there and not down where it could do some good. There's going to be cheap windows that leak and very thin walls will little more then an R-11 if your lucky. Even the ext. doors are going to be thinner. A normal house door is 1-3/4 thick. Most mobil homes are 1-1/2 or less.
I've owned several mobil homes before and unless you buy a better brand of home there made as cheap as they can get away with.
My last one was a Marlette brand and it had 10" of insulation then the ceiling, and real 2 X 4 walls with R-13. It even had real double hung vinyl windows.
It cost 1/2 as much to heat as a Champion and an Oakwood I owned.