Having powered a dozen buses with full time AC from Solar, I have learned a few things. Here are practical numbers from what to expect as you try to keep a bus cool in hot weather!
A 37’ Flat Nose Skoolie is less than 300 sq feet. That’s going to be 37’ Bumper to bumper and around 34’ behind drivers seat.
A 12,000 BTU Minisplit is rated for 200-450 sq feet.
Is this enough to cool the entire Skoolie? No.
Minisplit ratings are based on Residential building which is far more sealed than a Skoolie. The closest you can the thermal envelop of a Bus to match that of a Home is by deleting all windows, skinning over the sides, and installing RV windows which are SEALED. A proper door that is insulated and SEALED. Sprayfoam is the most effective in both sealing and insulating BUT, just 1/2” or 1” of foam is NOT enough and frankly, a waste of money. I’ve heard the argument that 1” of foam is the same as 3” of foam and/or the additional cost isn’t worth the gains. In residential, you do TWICE the insulation value in the ceiling vs the walls. As most thermal loss is through ceiling. If you do a thin sprayfoam job instead of a proper sprayfoam job, that $500 you saved will cost you $1500 MINIMUM in Solar and Battery costs to achieve the same Solar sustainability. Anyone who tells you otherwise is selling you lies or is ignorant.
A 12,000 BTU Minisplit can pull around 1,100w for cold air and even higher when using heat…but in most circumstances you won’t see these numbers. The draw is dynamic so that number is not useful for system design anyhow.
You can see in this three minute video that you aren’t going to cool a 300 sq foot bus with a 12k Split, even if parked in the shade in moderately warm weather. Most don’t realize that 70 and sunny outside = 80+ inside the bus. If your goal is to achieve full time AC in weather above 75, you MUST focus on insulation and in larger buses, you need 2x splits. For a well insulated bus with 25’ livable, you MAY get away with a single 12k and a fan moving the air from one end to the other, until you are in the southeast in the middle of summer (Not advisable 
)!