There is more to this than what meets the eye.
Most trailer frames are "cambered".
This is running weld beads at specific locations and patterns on the frame rails. This process slightly bends the frame, and more importantly strengthens it while adding a springyness to it. This allows for a lighter frame to be used, as the cambering puts the strong points just where it is needed. The locations/type of cambering are determined by the engineers,based on the expected loads for each trailers design. I became somewhat educated on this when having a sidebar dialog with Jbarca on a frame repair project he was undertaking. Curious about what I was reading, I grabbed some 10' metal stock from my pile and sparked up the welder. It was very enlightening. Each weld bead I ran caused measurable movements in the straightness of the stock compared to an identical unwelded control piece. More importantly the feel of the cambered stock changed dramactically. It felt more alive, with a definant springness that made it want to bounce back to its unloaded state as load was applied to it. The control piece felt dead and weak in comparisson. with out the "bounce back property of the cambered piece. IOW, applying enough force to permanently bend it was much easier.
As to what this means to what you plan on doing: I would only apply such force at a location that it was designed to take it. A jack as close as possible to the wheels should be pretty safe. Other than that, is a roll of the dice. You could end up applying force in a direction where the cambering is in the wrong direction to the force you are applying. Nothing good could come of that.
Your frames cambering can be in different locations and directions, and can be impossible to see once the trailer is built.
