RV Airconditioner
I spent over 15 years in the A/C business and what I'm about to say applies to any electric A/C unit.
The temperature difference you get across the evaporator will vary indirectly to the relative humidity. The more humid the air is, the more effort the A/C puts into drying it. Once the air is dry, the temperature will start to drop.
Let the unit run for at least 5 minutes with the indoor temperature above 75 degrees. If the evaporator coil is accessable, look for a full pattern of sweat on the coil. Ideally, it should go back to the compressor. Frost is bad. It means that the air entering the unit is too cool, the air flow is restricted (usually dirt), or a real problem.
Archer is dead on about the filters. As with any type of refrigeratoin unit the best thing you can do is to KEEP IT CLEAN.
It should go without saying, but don't put you hands mear moving parts or live electrical connections...if you're not sure, pull the plug and wail until everything has stopped first.
Check your onwer's manual to see if the fan motor needs oil. Most don't, but some may. The manual is also the best place to find out how to remove the cover for cleaning with doing damage.