My concern is that the jacks are typically mounted parallel to the bumper. Jack that thing up very far and if the RV does start to move in either direction, forward or backward, the jack itself would "roll" the RV the direction it is already traveling, and end up dropping the rv, ruining the jacks, and likely doing damage to the RV in the process.
Mounting the jack on the frame is problematic because (at least on the ford e450) the frame itself is narrow and not close to the edge of the RV. It would be a PITA to get at the jack to do the cranking. It just seems that all other things considered, using them as intended is best, while using the typical leveling blocks to get you close to level.
I have a small 19' 5th wheel Scamp. It has front landing gear actually designed to hold the weight. Even those, if I extend them too far, will "wobble" or flex, mostly side to side. That's my 2c.