Sounds like you figured out the second pic is whats left of the drain valve. If you replace Id try to figure out some kind of guard, or maybe a recessed can like a water fill valve (or reroute to rear)valve stuck on the side of wing like that seems accident waiting to happen.
Cant offer suggestion on the tank. As noted most off the shelf stuff wont stick. On the temporary plug maybe a rubber expansion plug available most hardware? Cant get orientation from pic but if its open/clear of camper when off loaded (water will drain without soaking camper) do you need the hose & remote valve?