hard to stop a small spider, but worse yet a mud dober wasp can sure make a mess as well, this is some things I did to mine........
on the vent on the roof for the frig, I used some plastic screen I cut in 2 to 3 inch wide strips how ever long I could work with? and used some enterabond tape to hold it on with around the louvered openings.
cant say how long the plastic screen will last? only been 6 months ago that I did it, but so far its been ok. frig has worked ok.
The water heater louvers I used a fine steel screen that i epoxy on from the inside of the metal door ya flip down, so far the heater seems to work ok with no abnormal heat from blocking air flow slightly with a screen. or at least I can tell that its running any hotter?
The heater exhaust and intake, I,m playing around with that a lil bit
I had a old grill tarp laying in the shop and decided to cut a piece of it about the size I wanted and went and got some velcro and industrial glue, I put the industrial glue on the velcro and on the tarp and on the camper and then stuck the velcro to both pieces. seemed to stick quite well. I than put my cover on, it seems to seal very well, stays on goin down the road very well, I also had some rubber window or door seal left in a package that was about 3/4" wide and put a small strip of that right above the top piece of velcro on the camper, used a lil bit of silicone to help it stay their to act as a over hang so water could hot get in between the velcro, just along the top.
so far I like it very well for stopping critters from getting in their.
However their is one thing about doing this that is not such a good idea?
if you happen to turn the heater on and do not have the cover removed, the heater may light? and the flames will not travel where they are designed to, I tried mine to see what it would do and it did light for a brief second before the flow switch kicked it out or the high limit kicked it out? so I made a small sign that is mounted to the wall above the thermostat that says DID YOU TAKE THE COVER OFF!!!!