Portable air conditioner produces nasty smell in fan mode after running AC
I'd be careful with cleaners, the manual online specifically says not to use any harsh cleaners. Personally, I'd use my judgment on this though. I'm not sure how much you can get at the insides, but I'd try and brush the coils with an old toothbrush and warm water with a drop of dish soap. 

I'd also see if I could clean the evaporation tank. Water condenses on the coils and drips down into the tank. Heat from running the unit should evaporate the water (just like your fridge) so it's normal not get much out of the drain hole. You should be able to take the lower drain out, spray warm water on the coils and see it drain out.  

You should also wash the air filter weekly, it's collects dust and not just lint, so visual lint isn't a useful indicator of anything. 

Personally, I'd never use the AC just in fan mode. If you want to just move air around, an actual fan is going to be more efficient. 

If these are the only two rental units left from your apt complex, you might see if they'd buy a new one, or consider buying a new one yourself. It looks like a model similar to this is around $400, so depending on what you pay to rent it, it might be easy to recoup that cost in a short amount of time.