Air bags would be my suggestion.
As far as a compressor, yes you can mount one inside under the back seats. You dont necessarily need a compressor though. On my first set of air bags I plumbed the air fittings back through the license plate. I then just used a hand bike tire pump to fill my bags to where i needed them as they dont take much air.
on my dads truck I installed a tool box in the bed of his truck, installed a tank and compressor in the tool box. I then installed air switches inside his truck where he was able to inflate and deflate the bags from in the cab with gauges on his pillar showing bag pressures and tank pressure. Also installed a set of train horns...
personally, i wouldn't recommend a leveling kit on a half ton towing a 5th wheel... I feel like you will end up regretting that in the end. The higher you go the less stable it will become. I have been there and done it, just my opinion.
