Since it’s a rental motorhome, and you can’t make upgrades, the only other thing I’ve seen done, besides running generator, is to install a temporary curtain behind the driver and passenger seats — much like a bulkhead. That can help by reducing the size of the cab to be cooled, but comes at a price. For starters it makes cab area feel enclosed, and also doesn’t work with other passengers or pets in back. Extra cooling in cab means less for the rest of the coach.
I tried it once on my Class C by hanging a clear curtain down from bed area but it wasn’t worth it because the A/C was good enough, so I didn’t even drive with it that way. I did use it to reduce A/C load once parked, but that’s a different problem than you’re asking.
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