The single-leg screw jacks which install across the trailer frame are compact, easy to use, permanently mounted, and add stability side-to-side, not just up-and-down.
BAL Light Trailer Stabilizing Jack
Camco Cross-Frame Stabilizing Jacks
I placed mine (the BAL units) not far behind the wheels of our Boler
1700 - not at the very back - so that they keep the trailer from moving up and down on the suspension.
These jacks can be placed angled instead (rather than straight across) to give better front-to-back stability as well - that's how I'll do them at the front corners if I ever add them.
uses telescoping landing gear at the front, just like most fifth-wheel trailers including my large non-moulded 5th wheel trailer. I would consider changing those to the single-leg style (but in a leveling jack, not just a stabilizing type) if I had to replace them, because they don't add any lateral stability - it's like supporting the trailer on stilts.