Just my two cents here but unless you plan to install stuff on the roof and the lowered antenna will be in the way, I would suggest leaving it in place and just re-sealing around the base flange with some Vulkem. Even if you don't plan to use the antenna, if you sell your trailer in the future a prospective buyer will most likely want to.
That said, if you are set on removing the antenna, after you do so, I suggest coiling up as much of the cable which was on the roof and stuffing it in the gap between the roof and ceiling skins. That way it is there should a future owner want to reinstall the antenna.
The patch on the roof could be as simple as a small piece of aluminum flashing that is "glued" into place with Vulklem. You could pop some olympic rivets into some of the original screw holes for the antenna base, but really the Vulkem alone should do the job.
For the interior hole, I would personally cut another piece of flashing as round as possible, and "glue" in into place with just a few dabs so it won't ooze out and be visible. After all the interior one won't need to be water tight. Once it is bonded in place you could paint that interior patch to match the ceiling as close to the original as possible, or maybe install another light fixture in the same location to cover the hole.