Your question reminds me of "Smokey and the Bandit" where Bandit takes more and more money saying "Faster than That."
Whatcha wanna spend? Many like GasMatics. More like Bilstein. A growing list likes Koni FSD. One Koni FSD probably costs as much as a set of four Monroes or a pair of Bilsteins.
"overload" shocks aren't likely to handle the weight of the rear of an RV for all that long or all that far. Your airbags are better for that.
When Rock (rockhillmanor) looks at this thread he'll tell you there's no substitute for replacing the rear springs. Says it'll solve sag and ease ride harshness. I'm sure he's right, but I'm not aware of many OP's who go to all that unless the spring is broken or severely sagged.
A number of us have done shock changes and upgrades. Others have worked with springs and air bags.
Now if your FRONT springs are sagged, those in-the-spring air bags don't help enough to be worth doing. Calls for replacement, and I'd suggest looking at SuperSprings.