Sharon, If you no longer have a need for the vent, you can probably seal it up with a rubber gasket from behind, across the entire vent, and then solve the leak problem.
If you still use the AC unit, you might try changing out the vent for a lockable access door that will keep out leaks
. Or a marine grade deck hatch. You just open the door/hatch when you use the AC. Line the inside of the opening with screening for bugs.
We all love our UHs so much that we view the leak issues as an 'irritating hiccup' of ownership. We seal things up (we think) and then chase down more
#&!@*!#% water drips!!
Happy and Dry Camping