The air conditioning load is tricky. Some people actually manage to run their A/C on a 2400 watt Yamaha or 3000W Honda inverter genset (but not in economy mode) and often do OK on a 3KW construction generator because of the different way it functions on overload.
However, the installation manuals for most RV air conditioners, 13,500 to 15,000 size, specify 3600 or 4000 watts as "minimum recommended" generator size. That's based on what the A/C manufacturer knows about the loads it creates, starting and running, and assumptions about other loads a RV will already put on the generator.
I would buy the parallel 2 KW sets, for the option of running only one, maybe even in economy mode, for light load boondocking, and having the pair available for running the A/C in our extremely hot weather, or the heat pump when it is running at it's operating limits, these 35-40 F nights.
You don't want to lug a 4KW genset around if most of the time you use a fraction of the capacity, and a 3KW is not just an awkward size, it is a pretty big purchase to find out whether it is actually big enough for the loads from your RV, in the conditions under which YOU will actually use it.
Tom Test
Itasca Spirit 29B