There is no need to cover the A/C unit unless you think wasps and mud-dabbers will build nests inside.
FWIW, I used to use a cheap farm tarp to cover our trailer when in storage. I used 2-3 times the number of elastic cords and those add-a-grommet things to keep it taut. A 20 X 40 farm tarp then cost about $45-$55 and the elastic cords were about $20 at a flea market. The tarp would last about 4 years, then sun-rot would take its toll. I never did discover any rubbed or worn spots on the trailer. That was a bunch cheaper than a custom RV cover.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"