You should only need Soft-start on third A/C to start compressor.
I’d run existing two first (already possible), set thermostat cold enough so that they stay on without cycling, and “then” start a Coleman high efficiency 13,500 with built-in Softstart
With nothing else on, Onan 5500 is about the same per unit as Honda 2200/1800 Watt which people power one 13,500 A/C. Considering cost and relative ease, I’d try that first but only after confirming two existing units are working at 100% of cooling rating.
Even if required, 3 new 13,500 efficient roof-top units should cost less than adapting 2 or 3 mini-split. Roof-top may not be as efficient over time, but cost of energy is almost a non-issue when only used occasionally like most motorhomes.
Other option is add 3rd air conditioner and power with a portable generator when off grid. Mounting a generator on hitch is extremely easy compared to mini splits.
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