Checking air conditioner. How to avoid stings from wasps ?

You might try encasing the A/C shroud with a garbage bag and spray insect killer under/into the bag. Let the spray work for awhile, remove bag, and observe.

If your A/C unit is perfectly sealed to the inside, the spray insect killer should not get into the interior of your trailer. The A/C is designed to recirculate interior air only.

Dusk and dawn is when wasps are on the nest and the least active.