So i have been researching to find out what a good R value for a schoolie would be for full timing it. i ran into this calculation to help me, hopefully it can help you as well.

take the area of the wall, floor or ceiling, multiple by the difference of the schoolies internal temp(or the temp you wish to maintain) and the current temperature outside then divide that total by the r-value of each surface.

Example: and 8 ft x 10 ft wall = 80 sq ft, the temp inside the bus is 70 and the outside temperature is 30 or a 40 degree difference, the R value we will call it 14 for this "wall"

(80 * 40)/14=229 btu's are required to maintain 70 degrees for that wall

a better example would be

24 ft by 3 feet for wall 1 and 2

(72 * 40)/14=206

206 * 2 as the both walls will be the same

so 412 BTU's for those walls

now lets assume you have a 26 ft by 10 feet wide roof/ceiling

(260 * 40)/14 =743 btu's

and for ease of math we will assume the floor takes up as much space so we just double that.

743 * 2 = 1,486 Btu's

1486 + 412 = 1898

1,898 BTU's needed to maintain the temp of 70 inside your schoolie when its 30 degrees outside with r 14 insulation. i grant these are brick and mortar numbers and you would probably need to add 10-15% more BTU's based on heat loss from windows and general heat loss. But it is still extremely manageable at 2,300 BTU's. A Mr. Heater portable buddy on 4k btus covers that easily with a box fan to move the heat around a bus.

How Insulation Works, What R-Value Means, and How to Calculate Heat Loss/Gain for your house | BrainStuff
is where i got my numbers.so hopefully this will help you plan and stay warm and toasty in your schoolie in the winter.