I bought a Viair Extreme duty on board air system. I am a little concerned about mounting the compressors under the truck.
I would be too.
Usually instructions provide you with mounting suggestions. Often they say something like "a clean dry location away from heat sources and where air can flow around it". 12V pumps work hard and create a lot of heat which needs to be dissipated or the pump will fail. That's why they usually come with a built in thermal cutoff. So inside a tightly enclosed space is not usually a good idea unless you can bring in cool air and vent hot air. It is also why 12V pumps that get used for long cycles often die young inside an engine compartment.
Maybe everyone mounts a pump under the vehicle, but to me it seems like a poor location. Why put a pump in a place where it might be submerged, dragged through mud or otherwise abused by the elements. I doubt if the manufacturer designed the pump for that location.
Maybe there is a place inside the vehicle where it could fit and not get in the way. Mine is in the rear corner next to the tail gate. Maybe for you it is best to leave it portable. Put it inside the lid of a box so it can have free air flow when removed.
Also don't forget that air pumps usually have an air filter that you need to remove and clean, so accessibility to the air filter or any other periodic maintenance points is a good idea.
Just spend some quality time with your rig and the pump, looking around for a place where they might both be happy together.