I believe you can adjust Timbrens to have almost no sag. They are typically adjusted to not effect the suspension unloaded. If the camper is going to be loaded basically full time, that shouldn't be a problem. They are a variable rate spring that usually has a gap. Buy the heavy-duty ones and adjust them so that there is no gap or even slightly preload them using spacers. I believe you can also use them in combination with upper Torklift Stable-loads to put some of the work on the overloads as well.
You can tell from my signature what I run, and I've found air suspension to be easily adjustable and relatively free from reliability issues for the past few RVs I've used including a Class A.