Find a 45 amp capacity generator and regulator for a Cadillac or an Olds 98, and don't bother with a second battery. Unless you're planning to run the air compressor with the engine stopped, there's no point whatsoever in using the deep cycle battery. The only real difference in a deep cycle is the fact it can be repeatedly discharged 75% instead of 25% like a normal battery without damage to the plates. That's important for a trolling motor or a golf cart, but not a vehicle battery.
Use a good-sized reserve tank, like a 30-lb. Freon tank with a one-way check valve, to give the compressor a little break.
Trailing arms and air bags will be a HUGE mistake, BTW. Add more leaves to the regular springs.
Jerry
