I know this isn't what you are asking for but we tried to do what you wanted and then gave-up because it blocked the front windows and required making a hole in the trailer. In the end, we created a mast using 1/2" EMT and attached it to the side front plexiglass cover on our 2016 FC 25FB which travelled over 14,000 miles without a problem. The stainless steel cable ties held it to the hinges of the plexiglass curved door but we used normal tie-wrap mounting blocks at the top to hold the truck booster portion onto the airstream. After 1-year, the glue held but the cushion part of the mounting block crumbled away. Moreover, it made it difficult to impossible to open the plexiglass to clean inside.
So we went the VHB tape route and used a couple of Stimpson elbows to mount the mast directly to the side of the trailer. I'll figure out later how to remove the remains of the old mounting block glue but here is how we did it using all Home Depot parts. Could have gotten fancier but I didn't have the metal working tools and I wanted a simple solution:
3 Simpson-Strong-Tie Model A21Z Galvanized Steel Angles
3 Halex 1/2" EMT two-hole straps
6 10-24 Stainless Steel Screws, Lock Washers & Nylon Locking Nuts
1 Halex 1/2" EMT Inside Corner Pull Elbow
10' of 1/2" EMT from which two pieces were made measuring 72" and 12"
Online I ordered 1" 3M VHB tape.
Per the photo, the Simpson elbows were drilled to accept the 2 hole straps. The EMT was assembled using the elbow and then everything was painted once with primer and then again with Seymour Alumi Blast paint. I used Alumi Blast because I have it for my car engine stuff but just paint to whatever color suits you.
I learned the hard way not to paint the area where the VHB tape was going to go because the paint peeled right off. So I used the VHB to peel off the paint from the other 2 elbows and then installed 3 pieces of fresh tape on the bare metal.
There will be one additional bracket as overkill also mounted with VHB to help support the oscillation of the antenna and serve as an emergency additional containment strap.
The wire was fished through the emergency egress window by making a tiny slit in the rubber gasket allowing the window to close securely over the wire. That wire was run inside the trim of the window, popping out by the electrical outlet by the TV, then through a hole in the wood allowing it to transverse the shower coming out above the TV in the lounge. This was designed to give the maximum distance from the mast to the TV that I could reach. When plugged-in, it boosts by 1-bar, sometimes two but if I upgraded the internal antenna I'd get more boost.