You could place some coarse wire mesh over the opening
to that vent. If it is small enough, it will keep out
everything but the critters that can get in somewhere
else anyway.
After being unpleasantly surprised by many changes
made by the PO, I have made a conscious effort to
change functional properties only where they enhance
performance. I think this approach will save me or a
future owner lots of time and aggravation. For
example, you never know when you might decide that you
want to do some winter camping. Then you'll be glad
that you don't have to worry about a cracked black
water tank.
Jon in SC
68 Overlander