Always mount stabilizer jacks to the frame (not the suspension). Mount them near the front and rear of the trailer for the most stability. If it's a longer trailer it might be a good idea to mount three sets instead of just two, with the third set being located NEAR the axle but still mounted to the frame not on a sprung member. This will remove the frame "bounce" that so many with longer trailers complain about. I assume you meant to replace the Lp with 30 lb. bottles so they will be standard size.
Good luck / Skip