You won't bend the frame on the trailer. You'll just bend the jack and break the caster wheel, almost immediately.
Trailer jacks are really only meant to hold the trailer tongue up. They are not designed for side loads imposed on them by pushing the trailer around, let alone being towed with the weight of the truck also on the jack.
Commercially available trailer jack casters are just cheap plastic wheels with stamped steel frames. No bearings. They won't roll unless the trailer is EXTREMELY light on the tongue.
Speaking from experience here. The only trailer I was able to use a caster on effectively is my single-axle 6x10, and then only when it's empty.
