Mac, this might be an idea...
When we had our rooftop a/c installed several years ago on our Trillium
5500 the guy that did the duty very cleverly took all of the weight
off the flat of the roof by fabricating a steel ladder that he suspended from the Trillium
"hump" in the roof. Yeah, it did involve drilling four holes in the overhead but in the three years since the install, we've had nary a leak nor a sag problem with a unit that weighs over 70 pounds.
Many times, a working solution can come from a combination of inspirational ideas. And while your trailer may or may not have the angled overhead that accommodates such a construct as ours, it's nonetheless offered here as grist for the idea-mill.