I'd be interested to know where you got the minimum of 40 psi from. Typical minimum pressures are in the range of 10 to 15 psi (at least for things like the Firestone Ride-Rites).
Anyhow, I find somewhere around 40 psi works pretty well for me.
It really depends on the particular unit and partly on individual preference what would be the ideal air pressure. The "correct" way to set it is to figure out the ride height of an unloaded or lightly loaded chassis, add the load, and then set the pressure so the ride height is about the same. Since you can't unload your motorhome without completely tearing it to pieces, that method doesn't really apply.
You can get some idea by watching the height change as you add or remove air. You can probably get a better idea by doing some test drives at various pressures--too much air, and it feels like you have cinder blocks for shocks; too little, and it kind of wallows around.