ZEP makes a stripper which should get it off. I would suggest that instead of removing it, that if you have removed the crystalized layer, you simply apply a fresh coat of the ZEP.
I previously applied 4-5 coats of mop and glo to the end caps of my MH and it looked great, along with the storage doors. After a year and bit it started to look weathered. I took some ammonia and tried to remove the stuff, and a) it took a long time, b) was a lot of work.
Well, a few weeks ago I decided to get the MH looking good, so I washed it, bleached it, did the Barkeepers Friend where needed, and then spent two days re-applying Mop & Glo to the end caps and this time to the entire unit. The areas where it was applied before looked good after just a couple of coats, the new sections took the normal 4-5. The next day it rained, all was good, but I didn't like the way the water ran off the unit. So I went over the mh with one of the many spray on / wipe off detailing sprays with UV protection and now all is right.
Just like wax on floors or normal car wax you have to reapply from time to time.
Tom Wilds
Blythewood, SC
2016 Newmar Baystar Sport 3004
2015 Jeep Wrangler 2dr HT