The OP's air conditioining may not be operating as a "closed system" ... in that the A/C's air intake is not 100% circulating only air from the house interior. In other words some of the A/C's intake air is continuously made up of some new (hot) outside air that is leaking in. This means that there is an "outside air leak" somewhere in the A/C's intake system.
If the OP's A/C system is otherwise operating properly with no intake air leaks and with all windows/vents closed and is 100% using "recirculated intake air" like it's supposed to be, then it's outlet air into the interior is always around 20 degrees lower than it's intake air. Since the intake air is supposed to be 100% coming from the interior of the house, it's outlet air temperature is always getting colder and colder until limited by the thermistat setting ... THAT IS IF the BTU capacity of the A/C system is great enough to, at any expected outside temperature, swamp out the continual heat gain from the outside through the ceiling, the walls, and the floors.
One should always buy a new RV with the maximum BTU size air conditioning system(s) offered - just in case they find themselves having to camp in unexpectedly high temperatures. IMHO, that's part of the "self-containment" goal of an RV.
BTW, a leak in the intake system would also explain why opening some windows would help with cooling from an air-leaking system. The slight pressure increase in the interior - as maintained by the air conditioning system's fan(s) trying to pull in some air from the outside, when they shouldn't be - would be relieved by the open window(s)r - thus permitting flow of some extra cooled air from the leak on out the open window(s).
