You betcha - drills use a lot of air. As I understand it, a 20 gallon tank is a touch on the small side for buiding an RV. You will still be able to build the plane its just that the compressor will run more often. I believe the recommended minimum size is 25 gallons. My 30 gallon comes on quite often when I use my drill.
As for the air coming out of the back, do you have more than one place the air is coming out of? Does it only come out when you're drilling or all the time?The outlet on my drill is at the base of the handle. If there is air coming out anywhere else on my drill, I'm not aware of it.