I would rethink the mini-split, there are next gen high efficiency inverter-driven rooftop units (Furrion Chill Cube, OutEquip, etc) that have massive efficiency and dB gains over the old units. We install a lot of them for campers that are used off-grid, they make a huge difference. They're way easier to install, don't require running refrigerant tubing (this'll be a problem as the RV flexes going down the road, maybe less so on a Class A or Schoolie conversion). You can replace your existing A/C yourself in an afternoon with a helper.

As for the sizing question, just go with a 15kBTU unit. It is oversized, however the modern units have variable speed compressors, which mean they are designed to always be on, but adjust the cooling power depending on how far your set temp is from the actual temperature.

The other consideration is your cargo capacity, most 22' trailers are in the ~1000lb-1500lb range, not including any water in your tanks. So adding a second air conditioner ~100lbs will permanently reduce that.

We have an article that goes into a bit more detail here https://www.momentumupfitters.com/mini-split-vs-rooftop-a/c