For the frig I've always hot glued fiberglass screening from the hardware store to the inside of the side and top vents.
I do the same for the openings under the gas bottles, and door louvers, or any opening that need to be protected.
Now's the time since everything is clean or will adhere without too much cleaning.
I've had the same screen hot glued on my 5thW for 13 years and about 50K miles, hasn't fallen off yet.
I've never bothered with the water heater. I open it gently to prevent surprises and use a vent brush on the burner tub if necessary.
That never took much effort and I'm not sure how standard window screening would hold up to the water heater exhaust.
I put Camco stainless furnace screens on both my current campers. There a bit pricey for what you get but they look nice, work well, and are easy to install.
I don't believe there much difference in any of the brands.