Remove the cowling...about a dozen screws, usually. Use your favorite dirt/degreaser/cleanser in a spray bottle and hit the grime. Wipe clean. Now you'll have a better view of things...check wire connections for tightness and cracking or fraying...clean the fan blades and lubricate the fan motor shaft...inspect tubing for any rust or coolant leak...spray the coil with foaming coil cleaner, available through any of the big box stores...let it do its thing and rinse thoroughly...make sure, at this point that drain holes are running clear...if there are cooling fins that are bent or flattened, use a small, wooden skewer or a coil comb to reset them...careful, they're fragile...now reinstall the cowling (you cleaned this as well, didn't you
). Go inside the trailer and open up the unit...inspect the foam filter...if dirty clean in warm, soapy water, rinse, dry and set aside...inspect duct work...you don't want cross ventilation leaks...reseal gaps with metal duct tape if necessary, then reinstall the filter. Using a flashlight take a peek into your ac ducts...make sure everything is well connected. You're done. Enjoy your revitalized unit.