just my experience and its limited so far:
my onan 4000 takes 1 gallon to 1.25 gallons per hour depending on load.
my family of 5+2 dogs rides in the coach. the cab a/c only cools the cab portion so we have to run the genset for a/c. while doing so, i can turn off the cab a/c and save gas by using only 1 a/c. the cab a/c affects gas consumption as it puts more engine load, especially while on steep grades. the genset's fuel consumption is fairly consistent. if the genset is on, then propane doesn't get used either (another expensive fuel).
i've ran my genset from home to destination and camped in the beach's parking lot for 6 hrs without any issues. when it got dark and time, genset went off as we didn't need any more AC power.
i don't see a problem with running the genset with a/c in a parking lot as long as you are allowed to run it... some parking lots may not allow it but if you do, all you are doing is consuming fuel so make sure you have plenty of it...
just my 2 cents..