oil film on a/c filter, what could be causing this?

Don't know how oil from the refrigerant could get on the supply filter. A refrigerant leak of any size will quickly empty your system and quit cooling.

What's the history of the unit? Did oil just suddenly appear after cleaning the filter for years with no accumulation?

Used unit first filter cleaning by you? New unit?

I'm also inclined to think cooking grease. Do you fry stuff on the stove? AC is probably pretty close. I've seen very greasy filters on range hoods.

If it is leaking from refrigerant system, it is not long for the world. Slight chance of a bit of leakage from overzealous lubrication of fan motor.

I'm still wondering, doesn't seem like this oily deposit would be from cooking, however if I pull the a/c cover off and run the a/c while I'm on top would I be able to see the problem if I have a leak?

ksg5000 wrote:
The only thing up top on my AC unit that has oil in it is the fan motor.

And that would be wrong. There is most definitely oil in solution with the refrigerant to lubricate the compressor. Which of course could only be the cause here if the system were leaking refrigerant from the evaporator coil. But since it sounds like this one is cooling ok I'll go with the others on this. It came from within...

And it'll help catch the dust. No problem.

RV ACs don't bring air in from outside, they recirculate the inside air, so as stated above it has to come from cooking I would think.

The only thing up top on my AC unit that has oil in it is the fan motor.

From frying bacon while the A/C was Running ?