here is a nice calculator for figuring out your heat loss.
Home Heat Loss Calculator
it was a bit complex, but once i got it figured out it was pretty easy, if you need help i'll be glad to assist.
however, all the time i spent on that last calculator can be simplified by an industry rule of thumb.
in general, you'll need about 1000 BTUs per foot of length of RV. so if you have a 40' bus, you'll want a 40K BTU furnace.
good luck