If you are not crossing a rivet line, indicates framing under the skin, between the antenna location and the place where you are putting the radio you should be able to drill a hole in the roof, from the outside, and "Fish" a stiff wire down through that hole to a hole in the inside skin behind the radio. Fishing take a bit of skill and patience. If you go Fishing tie a string to the end of the wire and once you have found the inside hole pull the wire and string into the trailer. Now use the string to pull your antenna wire up to the roof. When attaching the string to the antenna wire make several clove hitches along the antenna wire and with electrical tape tape the string to the end of the antenna wire so it is a smooth tapering of the electrical tape. This will reduce the likely hood of catching end of the antenna wire in the insulation as you pull it through.
The newer trailer use 2 sided foam to mount the skin to the horizontal framing on the sides of the trailer. I do not think tape has replaced the rivets above the windows so check out the rivet lines to know where the framing is.
If there is a rivet line between the 2 locations I would say short of removing the inner skin or drilling all the way through and running the wire on the inside of the trailer it is out of the question.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles