You can pretty much eliminate air usage if you manually adjust pressure to suit load only as needed.
If you do want a leveling valve choose one with a delay, so it takes an average and uses less air than one that adjust air pressure for every pot hole and bottle cap in the road.
I think you'd be fine with one battery so long as the compressor does not run with the engine off.
If you must have two batteries normal practice it to use two identical size and age in parallel. Unless they're somehow isolated from each other.
