If I have to extend the legs very far to level front to back, I'll build up some cribbing using some treated 4 x 4 x 18 with a piece of 2 x 6/8 or 10 or a piece of 4x6 on top. Since the current trailer has a single motor landing gear (Lippert), I'll then shim to within about a half inch. If it had dual motors like our last 5er, then you can bring the legs down individually to contact then raise with both motors (Atwood L.G.). If we had opted for 'self leveling', I would still do it the same way as I described. I also put wood or cribbing under the rear stabilizers too, use BAL chocks and some ground level chocks in front and back of tires
Oh and yes, those long leg extensions will safely hold the 5er - but it will wiggle