Come back and update in a couple months and let us know how that FlexSteel paint works out.
I repaianted the propane tank cover on my Outback TT with Professional Rustolium. I spray painted over the horrible fading brown and repainted with black. It's now been 2 months, but the paint is holding just fine. Although, that black REALLY shows the dirt. But the black also goes very well with my camper. I may switch colors eventually.
The cover on my air conditioner feels to me, to be the same material as my propane tank cover (that plastic vinyl stuff.) And I've used Rustolium on other plastics before too (as well as a lot of metal). It's always held up good over time on all surfaces. So wondering how that FlexSteel will work out.