When we put the very first kit on our truck, we had to beat the thing on with a hammer the frame was tweaked so bad. Once it was on, and we torqued everything down it drew down the frame to the frame support a bit and held a straighter position. The only real way to fix a frame is to have it straightened, or better yet replaced. But I have seen it help realign peoples frame better than not having the kit at all.
Here are some pics to show what I mean on a customers truck.
Here is a pic of my truck before RPG existed. Notice the V in the Bed.
Here is pic after stage4+ and frame support installed.
I think the reason is the V was reduced is because when we installed the kit, we supported from the rear tow hooks via jack stands. We had to do this because we needed to have the weight of the truck bend the frame back to as normal as possible to get the support on. When we lowered it back down, the V wasn't as bad as before. So here is what I think happened. The support acted almost as a splint so to speak and helped hold the frame in the same spot as when we lowered it down.
Hope this makes sense.