15K BTU is not enough for a 40 footer. You need 24K to 30K as a minimum. Even more in areas with 100 degree plus weather.
Your 15K unit probably needs 15-16 amps, and the 5K units about 5 amps each. A 30 amp circuit is rated for 30 amps MAXIMUM, but only 24 amps long term (continuous). You could probably Just squeek by with the pair of of 5Ks and the 15K if you are using NOTHING ELSE that draws 120 Volts. That includes the converter, so you would need to shut it down when all 3 units are operating and operate your 12 volt loads on battery.
2015 T12RBST Flagstaff Hardside
2008 Itasca Meridian 37H