The total weight would be the sum of the individual weights only if you keep the wheels on a level while weighing. If one is higher than the others, the numbers will be off. (More accurately, if the relative levels of the wheels changes between weighings, they will be off.)
There are two reasons for this, both boiling down to weight shifting. The first is that any leaning of the motorhome will shift the center of gravity towards and away from different wheels. The second is that the springs in the suspension when compressed have more force exerted (and hence less on other wheels). As these two tend to work in opposite directions, at least much of the time, it's hard to say exactly how the weighings will be affected.
I am quite satisfied with the ride-rites on my motorhome. They probably won't require checking quite as often as you're suggesting (but it would be wise to be on top of that until you know for sure how much they tend to leak down), and they help stiffen and stabilize the rear suspension besides keeping the ride height at a good level. Since you can't measure the unloaded ride height to know where to adjust them, it's somewhat of a guessing game. You'll probably be best off with more than 20 pounds; I find 40 is about right for my motorhome.
It may well be that the front is riding a little lower than it really ought to since it's fairly heavily loaded too and it's possible the springs have fatigued over the years. Assuming you have the twin I beam suspension, it is a good idea to get the ride height about right in the front because the wheel camber is partly a function of ride height and if it wallows too much it can cause the inside of the front tires to wear excessively.
