i do not know what dicor 501 is...
if it were my trailer i think i would use good old fashioned vulkem. AND a gasket!
when redoing the vent caps on rickstreams 74 excella we found the vulkem was still pliable. i believe they had never been worked on due to the condition of the screws holding them on.
so no, they should last considerably more than two years. i believe andy suggests replacement every ten years.
perhaps your vent pipes were not properly trimmed, not allowing the caps to seat fully.
at very least consider yourself lucky you don't need to remove plain steel screws that had rusted into place after 30 some years.
we broke then ground out or drilled every one! on three caps!
you call them ferrets, i call them weasels.