I bought a used Viair 300P off ebay for a good price. Then modified it to operate like a "normal" compressor by adding a pressure switch, gauge, quick release fitting and a new poly hose. The 300P has a higher cfm rating (2.3) compared to the 450P but lower duty cycle. I cut off the battery clamps and used a weatherproof marine trolling plug & recept. The recept. is mounted on the junction box in the A-frame (another mod.). I can reach all tires on the truck & TT with the hose.
The thing about 12 volt portable compressors is that they have a duty cycle rating that needs to be observed. Viair now has performance data on their website which helps. I didn't like the Viair curly hose and their tire inflator is junk.
I tested the 300P on a rear tire on our F250 that I run at 80 psi for towing. Can't remember the time, but it didn't take all that long to go from 60-80 psi.
The Q Industries MV-50 gets good reviews and is basically a knockoff of Viair, but cheapened up a bit. Both these are popular with off-roaders and their big tires. The 120 volt compressors are an option but are very loud and of course need 120 VAC - no good if out on the road or during CG quiet hours (like heading out early on another leg of a trip). If looking at 12 volt compressors, make sure there is full/proper data on psi/cfm & duty cycle.
