We have Coleman Mach 15 as our single A/C, not Ducted. Also have MaxxAir's nice heavy duty shroud on the rooftop part of that Mach.
Related to Pool Noodle, I found that the required Foam Baffling for the two edges of the Condenser (coil you see from the rear) didn't migrate to the Mach when the Shroud did. TuffMaxx or something it's called, good solid shroud!
Anyhow, Hot Air was being drawn back into the condenser. A/C overheating and shutting down. Wouldn't overheat with the Shroud off. I even made a temporary "roof" to cover the unit leaving the sides open. Chris Bryant warned me about the Foam Baffles and out came Pool Noodle and Pocket Knife. Problem Solved!
Many Many Many RV's waste their A/C in Bad Ducting. Almost makes me glad we don't have it. Investigate 1. Design, 2. Installation and 3. Deterioration. Then see if you have adequate cooling.
Fifths have a lot of "cube" inside, so 13.5 is probably marginal. When you serviced it, did you clean the "indoor coil," the one in the Front of the A/C, that you have to take some internal shrouding off to access?