Absolutely right. I've been really surprised to see many builders meticulously insulating their walls, floor, and ceiling and then leave the cab essentially uninsulated and expect to manage their heat gain with fans. Here's a little understood fact about R values. Equal square footage of R-1 and R-10 give you a blended R value of 1.8. That's because what you average are the U-values. So U of 1 and U of .1 average to 1.1/2 = .55 with and R-value of 1/.55 or 1.8. AND, then if you let solar rays actually enter the van, you have to deal with radiant heat from the sun. What you have to do is isolate your temperature controlled area and surround it with ambient temperature air using fans and then insulate to a high level like R-9. Very few people are doing this. In your case, if it's possible, you might want to first use windshield heat deflectors, then run a fan in the cab to exhaust the super hot air, and then a thermal curtain between the cab and your well-insulated areas. I would bet your experience will improve with that.I was thinking you were really about to be dissappointed until I got to the last part of your post. My experience with my well insulated van is that the 5000 BTU air conditioner can just barely handle it when the temperature is 100 degrees and 70-80% humidity when I am parked in partial sun.
The solar load is very important here. In theory you can insulate really well, but if you are parked in the sun that is a significant additional thermal load that has to be accounted for. White vans are better for this, but they all have metal heat conducting ribs. I have added some additional insulation in some critical areas since my last test, but the van will never be perfectly insulated. I have r10 in a lot of areas, but I still have the cab area with windows (which if not covered is basiacally a greenhouse capturing heat).