I just read a spec sheet on RVUSA that puts it at 15,000 BTU's. Also that 20 degree theory is referring to what the estimated temp drop across the evaporator as a "Rule of Thumb" from the "old days" of testing the capacity of an AC (18 - 22 degs). This is how it was taught back in the 60's when I went through New England Institute of Technology at its original location on Early St., Providence, RI. There modern way to determine if the unit is working properly is by taking temps at several locations, amperage, and comparing on a chart for that unit.
But with 100 deg temps outside, beating down on your roof and walls, it will be tough to get the temp comfortable. My rig is just 29.5 foot and has a 13,500 btu AC. In temps that high it may get to the 80's inside, but when the sun goes down, or on cooler days, or if parked in the shade I can get the rig down below 50's inside with outdoor temps in the 80's. It's a matter of heat infiltration which is dependant on the insulating value of the installed insulation. On those blistering days I keep all my windows blinds closed, cook less inside, extend my awning, and drink a lot of beer.