AC filter in mobile home question

Hey folks,
I'm in a doublewide where the AC unit has been running, but I have some questions on filters. The home was bought new. Please pardon my ignorance on this subject, but no one told me much about this unit or what i needed to do to maintain it. I fully expect that I haven't been doing all I should.
First, it is an air/heat unit fully enclosed. That is, it's next to the laundry room, without any doors into it or the water heater. The wallboard for the water heater had screws to remove it for access, but issues with water led me to remove this panel for easier access. The only other access is through a grill of sorts in the next panel. This is where I can see the a-coil I think its called.
Our home is sufficiently cooled I think, but there is a great deal of dust in the laundry room, and "dust bunnies" in and around the AC unit. The "grill" does not have any kind of filter behind it. The A- coil itself has two filters that we clean periodically. But I think the entire a-coil really needs cleaned and new filters need to be put on it.
Now I have noticed that only a few ducts in the house itself are really providing any cooling, and the AC will run indefinitely unless we turn it off. It doesn't seem to pay attention to the settings on the thermostat anymore.
I'm pretty sure Ive done something wrong...something needs cleaned, and I'm missing a filter somewhere. I think the "grill" into the wall should have a filter as this is where air is being pulled in from it seems. Ive seen these filters at places like walmart, but no idea if I needed one, or where it should go.
Yes, I have no clue. Can someone tell me what I need to do adequately care for this system? I surely don't want high repair bills or even high maintenance bills. A nice "you need to do this" from the manufacturer would have been helpful. Where can I buy these metallic a-coil filters, how do I check and clean the floor registers, and do I need an additional filter for the intake?
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