Air Conditioner

Ours the roof slopes starting about 2" from hole. Important is the hole itself if flat. But the end of our AC is a almost an inch off roof. Gasket kit included a strip for rear support, as I recall I moved it closer to hole so just shy of contacting roof (like it would if roof flat) before compressing gasket.
Our original AC was larger, the rear support strip didn't contact roof at all and it didn't leak or other ill effects from lack of support after 10 years.

View showing roof slope

Not best picture, just showing start of roof transition from hole.

Its the 4 mount bolts that hold AC to roof, compressing gasket-
I don't 'think' non-supported rear could cause a leak. I just didn't like of idea of it hanging and bouncing down the road. Possibly- likely wouldn't move though. If you added something just ensure it doesn't hold the unit up from compressing the gasket.

I installed the Polar Cub on my Bigfoot. It sandwiches the roof between the inner mounting plate and the A/C frame. It only needs to seal where the foam gasket contacts the roof. The Bigfoot roof is curved, so side to side the A/C really only touches where the hole is plus about 4-inches.

Wayne

Can you build it up with composite lumber or something similar?

I'm curious as well. Considering installing a low profile Dometic unit and my rear vent is labelled 'AC prep'.

Could you post some photos so we can see what the situation looks like.