I used stack jacks for long term snow birding in Fl on a previous 5th wheel. IMO, the best place for them is in front of and behind the axles under the frame. I had some brackets made at a local fab shop that went between the jack and the frame to keep the jack from slipping. The bracket was made from "C" channel just wide enough to span the frame with a short piece of pipe welded to the back. The pipe was sized to just fit over the round cup at the top of the screw of the jack.
2015 Rockwood Ultra Lite 2604 WS. Rear LR, large street side slide with small wardrobe slide in BR.
2014 Ram 1500 Eco Diesel Laramie Quad Cab.