You can convert an old Scuba tank for this and it's a lot smaller and it will fill a LOT more tires then a 20 gallon tank will. You charge them with CO2.
Here is a company that seels them ready made. http://www.powertank.com/
You can do the same thien with a soda fountain CO tank. I have several friends that have built these for use offroading. They can air up about 50 large floatation tires ( read as LOTS of volume) off a Fountain CO tank. My off road truck has some tank I built into it that hold about 8 gallons of water. There is a conversion for Air gallons at a certain PSI. I can't get two 35 inch tires from 20 psi back up to 32. The power tank would tak up 1/4 the physical space and do 50. This is the same style set ups pit crews for Nascar use but they use Nitrogen.
The tank is small enough that you might be able to tuck it behind the two propane tanks.