I don't really notice any difference in gph with the gen on compared to the dash air. I suspect the gen uses a little more fuel than the dash air but, like you, the dash air just does not cut it in Texas. We live in East Texas where the humidity is very high (mold molds here). Even at about 80 degrees the dash air simply cannot remove enough water from the air to make it half way comfortable. I run at least the front air and refer and about 30 to 45 minutes from stopping for the night I turn on the rear AC. A hot coach takes too long to cool down in Texas and want the head start.
I've read the posts as well claiming the roof air uses less fuel than the dash air and think most folks are rationalizing and feeling guilty about running the gen. Subjective at best. I also heard that the dash air robs 10% of the power but that would be 40 hp with coaches with an ISL and 40 hp equates to about 30,000 watts (750 watts per hp).
Dawn and Mark
06 HR Endeavor 40 PET