the two existing units provides 30k Btu, your thinking of a multi split of 36k Btu it will in theory give you 6k Btu more . if you think that 6k will bring down the coach temp to your liking, I wish you good luck. the original design is meant for brining down the temp to a differential of 15F outdoor to indoor so if your are in an environment of over 100F and your indoor temp at 87F is pretty close to the units capacities.
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