Maintaining a comfortable temperature inside an RV is a significant energy problem. The RV is often exposed to the elements and cannot devote a lot of space to insulation or machinery.
The rule of thumb for furnaces is that an RV needs a furnace that will provide 1000 BTU per hour of heating for each foot of length. Air Conditioners provide about half that for cooling but then heating often needs to maintain 40 degrees of temperature difference (from 20F to 60F) while cooling just 20 (from 100F to 80F).
An average office building uses 92 kBTU/sq foot/ year (energy metrics). For an 8×30 foot space like an RV, that’d be 23,552 kBTU/year or 64.5/day or about 2.7 kBTU/hr.
These guidelines provide a range from a peak need of about 30k BTU BTU per hour to a bit less than 3 on average. The ORNL biomass conversion factors can provide an idea about what that means in terms of energy sources. (they also provide data to help you calculate the carbon footprint if you are into that)
See convert units for a very nice calculator for distances, units conversion, and other things.
For Energy: Wood has about 6400 BTU/lb, gasoline 115k BTU/gallon, propane 93k BTU/gallon, and a typical RV battery 3400 BTU (but only half usable). What this means is than an RV might use each day, on average, ten pounds of wood, a half gallon of gasoline or a bit more than that of propane, or twenty batteries worth of stored energy.
For Peak Power: The 30k BTU/hr peak power would mean a consumption of five pounds of wood per hour, a third of a gallon or propane or a bit less gasoline per hour, or ten batteries of energy each hour.
Solar is sometimes sold as an ultimate solution. Consider that the absolute maximum power from this source is 3k BTU per hour per square yard (SMPS – updated 2/23/2013 re Lazar Rozenblat). With a typical 10% panel efficiency this means about 350 BTU per hour from a standard RV type solar panel. That means nearly ten panels would be needed to supply peak power needs and that would only happen with optimal sunlight conditions on the panels.
Note that these calculations are very rough. An RV isn’t an office building and the peak needs don’t consider extreme circumstances or efficiency factors. See the ORNL page for some idea about how these can be considered. Check the arithmetic! The numbers seem reasonable but errors can happen and sometimes reasonable isn’t reality. Peruse the references, find other resources to compare, check the arithmetic and make other comparisons. You’ll gain an understanding about why things are done the way they are and be able to compare your needs to your options better.