I just finished installing and piping the 3 gallon tank from Siege Engineering, (E-Bay) I valved it off with a 1/2" Ball valve, tied it into my CO2 tank and air compressor system. If any of you are serious about On-Board air, I suggest a CO2 tank. My 5# is a 320 Gallon capacity, around 1200# regulated to whatever you want. Mine is set at 120#. You can get the entire set up for around $300-$400.00. Or, if your Brother-in-law has a bar, or some kind of eatery, you can talk him out of a 20# CO2 tank, take it to a welding supply company and trade it for two 5# tanks, sell one to a fellow RV'er and buy a compound gage. You will be in business.
I have my 12 volt compressor valved so I can use either or both.
(Hit the slide show button and set the timer to 5 seconds for better viewing.)
Here are the pictures: