I was neither clear enough nor explicit enough. The butyl tape is for under the belly band trim, where it seals equally as well as it does under the window trim and roof vent trim, etc. It's not for the little cracks at the top and bottom of the bely band trim after it's on. We had ample silicone sealant along those edges thanks to a PO's ignorant exuberance, and of course it leaked. We had to pull the whole belly band trim and clean and seal it properly, with butyl tape. Just like we sealed all the windows
. After a year now, not a drop of leakage anywhere.
Glen Baglo has the perfect idea, rinse it out once in a while if you're so concerned, though you don't have to rinse out around the windows
; nothing will stay in there, it's full of butyl tape. But, any loose dirt will get washed away, the rest is irrelevant.
You don't need to seal the belly band EXTERIOR where it sits on top of the fiberglass. But you have to be sure it's not leaking UNDER the belly band trim into the trailer.
If we had it to do over (and someday we might) we would FIBERGLASS the top to the bottom and to heck with the original bellyband crack. Then we'd still use a little butyl under the trim, and we still would not try to seal that little bit on the OUTSIDE. Though there may be a good reason why these trailers weren't fiberglassed together into one cohesive unit--I understand they had to be in two parts to be made originally. But now? r
Silicone on fiberglass WILL pull away, as you say, peel off, and it will leak.
If bathroom sealant is butyl based, it will work FINE. If it is silicone based, it will pull away and let dirt and water inside. Then once again you're t the mercy of whatever sealant is UNDER the belly band.
Have you considered that factory sealant was butyl-based? It can look a lot like silicone but over time it behaves quite differently. And that's not to say that a lot of silicone won't stick--it sure will. It's just that it doesn't stick everywhere, or well. Where it does stick it's a bug to get off.
YMMV applies to many things, but not to the question of silicone sealant on fiberglass. It's just not a great idea.
But of course, everyone is free to do exactly as they like, including ignoring advice they don't agree with.
PS. In 2016 we bought butyl tape in black, gray, and white. Yes, we bought white butyl tape. But we didn't use it to seal tiny exterior trim cracks. If you have proper butyl tape under
the trim, you don't need to seal those exterior bits.
YMMV, as always.