My problem child vent was on the rear wall of our TT. Plastic pin hinges at top, and two plastic latch tabs at bottom, covered by a separate part - the hood. Can stand on the bumper, but often forgot to latch it.
One day, it became gone. Then we started getting back drafting and dust all over the range top.
Replaced (affordably) just the door flap. No more dust, no more packing (which we also used for awhile).
So a missing flap CAN be a problem.
For a side mount (TC), maybe no so big a deal. I like the awning hook or dowel idea expressed by some.