All three of the 5th wheel trailers I've owned were rock solid and weren't sitting on concrete blocks.
I use various size blocks under the landing gear which is determined by the type of terrain were camping on. Some times the pin is waist high or sometimes its well over my head after sitting up.
As you can tell we don't camp in many RV parks with leveled/flat concrete pads and I also carry a big supply wood for leveling/blocking in any terrain.
JTs would be nice but I find my '80s era aluminum bi pod pin stabilizer at the front and BAL telescoping stabilizers at the rear take side to side/up and down motion out. I also use adjustable chocks between the tires that eliminates fore and aft movement. I also use blocks under the landing gear pads and the rear BALs. I'm rock solid for my needs.
