A truck camper with no propane vent for 5 years

If the access didn't go into the interior, why would it need a screen to keep critters out?

If the critters could get in and cause damage, then water would also get in. Water tends to pool & spread wherever it goes.

Maybe the water pooled at the lowest point, causing rot. The floor may be very soft. When we were looking at used fifth wheels, saw many with rotted floors due to water seaping in at an access hatch and then pooling at the low point.