They are accessed by the roof. Take the cover off (on the roof).
If you have a standard Coleman type A/C it should have a plenum on it. That is the cover (inside the trailer on the ceiling) that can be removed. Usually just clips on. Then you can access the inside where the 2 channels are located (one is the return, one is the out. This is the the thing that is holding that sponge material air filter. If yours is completely ducted with no plenum, than the only way to access the under side is by removing it from the roof.
As stated above, they are anchored down on the roof.
Here's a link to a bunch of YouTube videos that you will find helpful. Watch a couple of them and you'll get the idea what "should" be done for customary "maintenance" on them. And yes, "more" is not good. If it is working and working fine, don't mess it.
Click here.
__________________
History is not about the past, it's an explanation of the present.
2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually