I had an ultralight that had a 29 ft box on it. The single AC would lower the temp in the camper 20 degrees from outside temp AT BEST. More often 10-15 degrees lower than outside temp...especially if the camper was sitting in the Texas sun.
I never got around to doing it before selling the camper, but had researched taking out the vent in the master bedroom area and putting a non-ducted unit there, to augment the 15k ducted system that was already in the RV. Article I read, they had run a separate 120VAC power to the camper for the second unit they added. I really see this as the best fix for the problem. I have seen the silver insulation in windows to keep the heat out. Seen the insulating pillow put in the roof vents to keep the heat out. But you pay the same for your camping location whether using one AC or two...and every power stand I saw offered 120VAC plugs in it also. So why not enjoy the view and the full features of your RV?
Sean