I had the same , or similar thing on my Hitchhiker water tank .
Tank vent was at the side on top , and would be water logged , venting no air as tank would continue to blow up like a balloon with a huge air pocket in the top center
It cracked at an inside corner at the top , because of the stress.
I pulled it out and had a shop weld the cracks and put a gusset on it
Also had them weld in a new fitting in the top center , which I tee`d into the existing vent line
I don't fill the tank until the vent line pucks water , I stop when the panel says its full , all good now