Vehicle color has a very large impact on how hot the interiors can get. A Ford dealer in Texas took measurements of a bunch of identical Ford Transits when sitting in the sun, and found that the Silver and White vans were the coolest (114-118 degrees) while the hottest was the dark grey (at 142 degrees). Other tests have come up with similar results.
Reflective sun shades for the windows can make a large difference, especially when paired with ventilation. A list of custom sun shade makers is found on the Insulation page.
The most practical way to run A/C is by shore power (plugged into the grid). Most R/Vs and vans designed for this will need a 30 amp connector.
The next easiest way is to run a generator. You will likely need something in the 3,000 watt range.
It's possible to run A/C off of a battery bank, but you're going to spend tens of thousands of dollars to make it happen. https://faroutride.com/12v-air-con/
This topic has its own page.