I did a little quick internet research on the subject of what size of screening you will need to keep out wasp and bees. The keyword term you need for knowing how to order screening in the various sizes of opening between wires is "mesh". A website on pest control stated that for bees you should purchase a #8 mesh. A #8 mesh has openings that are .1000".
The next thing to know is the diameter or the wire you want to use. The smaller diameter wire will be easier to form, the larger the diameter the stiffer your screen will be. A wire diameter of 0.0250" would work nicely on the inside of a fridge vent, be easier to form and the openings in the screen will be large enough not to have a significant restriction on the air flow. If it was going to be one of the screens they put on the exterior over the exhaust from a furnace you would want a heavier wire such as 0.0320.You will also likely see some alloy numbers which would be 304 or 316, they are both good and appropriate to use but 316 is somewhat more corrosion resistant than 304.
As to mounting the screen behind a fridge vent. If you want to attach the wire directly to the back side of the vent cover then polyurethane caulk will work very nicely for an adhesive. It is excellent for adhesion on an irregular surface area and will hold up to heat, cold and the expansion and contraction of materials.
If you can't for some reason attach it to the backside of the vent then cut the screen slightly smaller that the cover but larger than the opening in the RV. Then put a layer of butyl tape onto the fiberglass
, gently push in the screen but not all the way in, then put on another layer of butyl tape and then put the cover on. Be sure to use an awl to move any wires out of the way of the screw holes before you try to drive the screw into the holes. With the screen sandwiched between the soft layers of tape everything will still be nice and watertight as all the gaps will fill in.