I wanted to add on-board air and had a space available underneath the bed at the rear of the Roadtrek 18" x 18" x 8". So, I put together a low profile compressor based on the ExtremeAire high output continuous duty 12V compressor. This model has a 3/4 hp motor and draws 50 amps. By mounting the compressor sideways and using a small 1 gallon air tank I can fit it in. I made it to be portable, and have a set of jumper cables with the mating winch 12V connector for use outside of the RV.
Cool setup! I ordered a compressor that is similar (or the same) as that from this place....they have the best prices on all tools (though shipping sometimes is slow) http://www.tooldiscounter.com/ItemDisplay.cfm?lookup=SUPMV-90
They have other models too: SuperFlow Air Compressor from Tool Discounter (http://www.tooldiscounter.com/catalog/SUP.htm)