I know a couple of ranchers who put gooseneck adapters on their fifth wheel RV, because they were towing gooseneck trailers most of the time, the fivers occasionally. Neither experienced frame failures on their RV, but neither pulled their RVs very often, very fast, or very far.
If you need to have a gooseneck ball hitch to tow other trailers, a better solution for a fifth wheel RV is to install a fifth wheel hitch on the gooseneck ball. This is an inconvenience if you have to remove and replace it frequently because you are towing several different trailers day to day. Thus why some folks use the adapters (and why the same ranchers don't often bother with weight distribution for conventional hitches).