When you start the task of leveling a truck, it is really similar to engine performance mods, a very slippery slope....and every change has consequences of it's own.
As cerb stated, level the front, then hook up a trailer, now you have to raise the rear(adjust headlamps) in some manner or your headlights will be blinding everyone. Raise the rear, need additional adjustments on ball mount, or 5th wheel hitch adjustments. May need to change spring pack placement on trailer to above the axle...now your stabilizers do not reach the ground without blocks, or your loading ramps are too steep. Sheesh! it's never ending!
I started off with a 2 1/2" Daystar level kit, then added airbags for above stated reason. Then I wanted 35" tires without rubbing...did a 4 1/2" Tuff Country lift, hooked up my 5th wheel, crap, even with airbags back end sagging, had to add 2" round spacers underneath the airbags. Now 5th wheel does not have sufficient clearance at the bed rails and does not tow level, had to install the Dexter spring pack kit to set axles underneath springs, you know the rest...
Here's a pic of my truck, I believe it looks nice, and it drives decent, even towing, but I sometimes wish I'd never slipped down that slope!
Jess