Measure the hole in the roof of your trailer. The "standard" AC hole should be 14" x 14". If your hole in the roof is larger than that, then the best way to correct the problem will be to remove your AC, then patch the hole, cut a 14x14 hole in the patch, and then reinstall the AC. If your hole is 14x14, but your AC is too small to cover the entire hole, then a new AC may be the best/simplest approach.
Is your AC the original unit (typically screwed or riveted to the roof with a million fasteners), or is it a more recent vintage (held to the roof by clamping the exterior unit to the inside ceiling. If it is a newer model, it could be that your AC has gotten loose, or was installed poorly and has slid to the side creating the gap.
Interior and exterior pics would be very helpful!