I've done all sorts of stuff there. Have you thought about a 2 or 3 foot hitch extension to move the trailer further back to allow room for the mast?

I've welded up and mounted steel mounts to the roof of my past RVs (wrapped the arms in rope to provide some padding). Cut small access holes in the ceiling skin to tighten the nuts and used milk jug caps to fill fill the ceiling holes lightly siliconed in place. Use plenty of 3M 5200 under the brackets outside if going this route. It's a bit of a pain heaving the mast up and down and also consider how the spreaders will stick out when considering other roof mounted objects and the outside perimeter of the RV (having the spreaders extend past the wall of the RV is probably not a great idea).

The best solution I came up with was to extend the tongue of my trailer. I added something like 6 or 8 feet and put in a new set of bracing on the sides of the main tongue member. It took a bit of work but it made everything super easy and avoided climbing up on the roof in the rain and so forth.