If rusty 22 gauge has held up fine. Then 14 gauge stainless would be strong enough.

The Galvinized is probably much better than originally used. Encased in fiberglass it will last for a LONG time. Until you drill a hole trough it to attach something, then expose that hole to salt, then all bets are off (Unless you are also careful to seal the hole). I'm just saying if I was going to all that trouble I'd spring a couple of extra bucks and use stainless. Yes it is weaker. But you are nearly doubling the thickness, so it would be fine. Not saying what you are doing is wrong, just that I'd do it a little different. Who knows, your way might be better - but I suspect it will take years of abuse of either to find out

Again, nice job. Got plans for a lift top camper