Swamp Cooler's only going to work in low humidity areas.
Are your A/C DUCTS OK? Pull your grilles and look both ways with flashlight and mirror. You may find disconnects, non-connects, bad baffling at the main A/C unit, etc. If you can get on the roof maybe you'd find cool spots indicating duct leaks.
If you're camping in the sun, you MUST cover the windshield, maybe shade or film the side windows. I suspect that full wall slide has more heat loss than smaller or no slides.
We camp in the FL summer in our 31-ft Class C, often in beach areas with no shade. We isolate our "Van Nose" with blankets, run fans, have tinted windows, and our 15000 A/C runs full time in the daylight. Will cycle at night. We have to shed electrical load to run that A/C constantly. Water Heater on LPG, Coffee OR Toast not both, all small appliances OFF to run Microwave. Our A/C is NOT ducted. Distribution isn't all that great, it's noisy, but we don't have any ducting losses.