Here's what I do:
First, pre cool the rig. The night before we leave, while it's plugged in at home, I leave the rig open to the cool night air, and then close it at sunup and turn on the roof air. Shut down the ac packs before you unplug.
Second, run the dash air on recirc. No need to try to cool outside air before it becomes inside air, and this keeps the road dust out of the rv. Rarely if ever run the dash air on fresh air. Fresh air is a car paradigm, your home ac runs on recirc.
Lastly, if dash air proves inadequate, fire up the genset, let it warm for 2 minutes, then run the front ac, the rear ac too if necessary. A small oscillating table fan also helps move the air to that greenhouse we call the cockpit. Rarely does it get this bad in my rig, we're talking Arizona in the summer.