Enough gas for a test drive isn't necessarily enough for the generator to run. The tank needs to be at least 1/4 full, possibly a little more.
As to the original issue, it sure sounds like a weak battery to me. Also being new doesn't always mean the battery is fully charged yet. Just this week my brother put a new battery in his daughters car - he had to charge it first as it was down on voltage, and it was a fresh build date too.
That said, I still don't blame you for wanting to hear it start and run. And isn't is sad we've gotten to the point we have to question people first and trust second? Unfortunately that's not getting any better either.
