We now have a 12 volt Viair comp. and it's a model that has their highest cfm in a portable. Nice that it's small and will fit in the underbed storage. I'm quite happy with it's capability.
Took our pancake comp. on a long trip last year. Was surprised to find how much psi changed from Reno out to the Ca. coast where it was cool and raining. What I've discovered is if overnighting and you want to add air to the tires in the CG before end of quiet time, a compressor is way too loud. If you try and find 120 volts just outside a CG, there's a good chance you may not find it. With a 12 volt compressor you can air up anywhere.
If you have to mount your spare tire, you could find it down on psi. You don't want to run tires under-inflated, esp. ST tires. I like to regularly check psi all around. You never know what can happen on a road trip. I have a small portable tank and it doesn't really hold that much air - friends borrowed it to air up 4 trailer tires about 5-10 psi each and had to refill it 3 times.