I know you said you checked the coils and they're clean. But since you have a new rig, have you had the air conditioning checked to be sure it's operating at optimum efficiency? Can you detect any duct leakage? Do you have a voltmeter or energy management system to be sure you're getting adequate voltage? We used to keep a 37' fifth wheel cool with one AC (15,000 btu). A couple of really hot days (100+) made it work pretty hard though.
Dave & Jo Ann
'08 HitchHiker Champagne 35LKRSB | '11 F350 Lariat 6.7PSD