I deal with this stuff on a daily basis. Rodents AND cabin filters.
Some vehicles don't have filters....just gets "stuff" in the evaporator and/or heater core box. Even with filters sometimes it happens.
Cleaning an evaporator is not always much fun!
I did an M9540 the other day, they have filters but the evap and heater cores were completely plugged (solid) with...DOG HAIR. Guy lets his buddy ride around with him, and that's fine, but the excess hair WILL plug things up, so the filters need to be checked often on stuff like this. My car...and truck...do not have a filter at all. The car's evap was plugged, a/c not working all that great, lack of airflow, etc. The evap is nearly impossible to get at. Had to pull the blower fan assembly out, then the resistor, and reach in from the blower side with a can of coil cleaner, then let it soak a while, spray it again, soak, then go in through the resistor hole with a modified air blow gun to blow the junk back through, then vacuum it out. Nasty stuff in there but it works 10x better now. Next project, do it on the project car, then the truck. The truck won't be any more "fun" but at least it's all accessible from under the hood instead of having to fumble around under the dash.
Did an L5030 cab a while back too, rats (or squirrels, whatever) got into it, built a nice little nest, and of course you know what they do in the nest. They pee and poop. So when owner turns the tractor on-and the a/c or heat-the fan blows the "stuff" into the heater or a/c cores. That thing STUNK to high heaven! Couple cans of coil cleaner, some labor, and lots of fine cleaning with a toothbrush, and it's good to go.
Speaking of a/c evaporators, on Kubota stuff, I see this all the time too. Someone's a/c don't blow as well as it used to, so the automatic response is "it needs a can of freon". They'll put a can or two in, plus leak fix, and it don't help. Call the expert (neighbor, shade-tree mechanic, whoever). He does the same thing. Still no dice. Last resort, call me. Tractor comes into the shop, I turn on the a/c, sure enough it's warm, or in a lot of cases low air flow out of the vents. Full a/c system cleaning....which is cleaning of the condenser, CAC (intercooler) if it has one, radiator, tighten the fan and/or compressor belt(s), then clean the evaporator, check the fan blades for dirt buildup, THEN look at the refrigerant. On the one, I saw low side pressures of 100+ psi and high side of 450+. So at that point, there is a question to what's been done to it. Make call to customer, get the history (hopefully it's accurate), then it either gets repaired, or in some cases components get replaced. If there's no leaks, it doesn't need freon and if a/c is inoperative, the other stuff needs to be looked at FIRST. If it does need freon, it's got a leak and it needs to be fixed. How do you know it needs freon? Gauges and knowledge. Unfortunately, not everyone has either one, and a lot of folks "think" they have it and shouldn't be within 2 counties of any a/c system.
It won't be long and R134A will be like R12, must have license to buy it. Already getting that way and it can't come soon enough! Must have license to buy 30lb.