Installing MaxxAir MaxxFan....Eternabond??

That should work fine. When I had a small leak on a seam bracket on the roof of my old trailer I removed all the original Dicor and replaced it with Eternabond. It worked great. Just remember to prep properly and you should never have to touch it again.

When I installed mine, I used Butyl tape under the fan flange and Dicor at the edge. Since I was putting vent covers over the fan it wasn't going to see the sunlight ever again so the Dicor will probably outlast me. If you really want overkill, do the Butyl tape, a thin bead of Dicor and cover the whole edge with wide Eternabond. Whoever buys the trailer next will either thank you or curse you. I put a piece of Eternabond over a small cut in the roof several years ago and it's still hanging in there.

I would install it like OEM. Use butyl tape and then add self leveling sealant such as Dicor.
If your 2014 trailer still has warranty left check with the dealer or trailer mfg first. Adding Eternabond to the new roof can void a roof warranty so one member found out.