We took a trip to southern Utah, temps 118° in Kanab - ugh. Running the Dash air and both roof top it was still warm. I had 2 fans blowing air from the back, helped a bit but still hot. When we parked and had our solar shade and curtains closed we could get the RV fairly cool, but not when traveling.
On another trip we did try hanging a thick shower curtain just behind the driver/passenger seats, dash air wasn't enough so we fired up the generator and had 2 of the air top vents pushing air up front and the dash. That helped but the dash just isn't powerful enough.
Reality is that it's just not well insulted enough and the front end with all that glass makes it worse. I suppose a stand alone might help, but we've just tried to avoid driving after 2PM in hot climates. I live in the desert so we've learned to adapt. What's the size of your roof top A/Cs?