Op, you should be fine if you pair your included dual ACs with a 12K mini-split. Or throw in a larger mini-split and you can turn one of your factory units off. I ran the calculations for our rig (in my sig) a while back and figured that we'd need around 42K BTU to keep it comfortable in the AZ summers. That means I'll be adding a 12K BTU mini-split at some point in our rig's life to the 2x 15K RV units we already have.
This is an example of one having been retrofitted.Dometic doesn't advertise SEER ratings, but I read they're somewhere around 9. By comparison, home AC units might be around 13-16, and mini-split units can be 18 to OVER 30! Higher is better.
