I have been using an Extremeoutback compressor for the last 15 years. It has been sitting under the truck the entire time. It is a sealed unit. I have it tucked up under the bed between the frame rail and the bedside so is fairly well protected and I also have a couple pieces of sheet metal to help protect it even more. I originally put it on my 01 then when I got my 17 I moved it over to it. I have a 2.5 gal tank mounted under there as well.
Large high pressure tires use a lot of air to fill them up. As mentioned get one with 100% duty cycle. A small tank will help but not that much. I have a quick connect at my rear bumper so I can reach all my truck tires and trailer tires with one of those coiled hoses. I use the compressor and tank for tires, air horns, filling my fishing float tube or anything else that might need an air source such as blowing out filters out in the field. A good one is not cheap and few will really run an air tool well but there are certainly some that can. Mine while pretty good is not capable of taking my tires off with an impact gun just not enough oomph.
this is the one I use https://www.extremeoutback.com/product/8/ExtremeAire+High+Output+12+Volt+Compressor+Part%23+007-000.html
Whenever it fails I would probably go with this one
https://www.extremeoutback.com/product/14/ExtremeAire+Magnum+12+Volt+Compressor++Part%23+007-222.html
The grandaddy of 12 volt is this one but so is the price
http://aircompressors.oasismfg.com/item/air-compressors/xd4000-continuous-duty-air-compressors/item-1052
An engine mounted one would be the best if you have the space
https://www.cuoffroad.com/product-lines/airboss-components/
Viar makes a good line of compressors as well
https://www.viaircorp.com/oba
I prefer to have 12 volt so I can use it anytime anywhere. If it was just for a motorhome I might feel different but I use my truck for more than just towing the fifth wheel so while I cant say I use my compressor all the time it gets used more often than my home compressor though my home compressor gets used for much longer and higher capacity needs that the 12 volt cant deal with with at all.
2016 Jayco 28.5 RLTS