The original manual for my '77 Boler says, approximately, "to change a tire, just use your car bumper jack (italics mine). Even at an RV service shop I prompted them to use their floor jack at the bumper. It works fine.
...Jack the trailer up using the frame. Usually the best place is right in front (or behind if you have a leading arm axle rather than trailing arm) the axle bracket.
Thanks, Roger! I always knew that I had to jack mine up behind the axle, but I never knew why. Now I do!