Goodman GMP075-3 Furnace - Performance Question

I've read up on the posts regardin goodman furnaces in general. OK its a contractor grade unit, and not top-of-the-line stuff. That being said...
My buddy is renting a house (Northeast-Us) and he just cruised thru what I consider way too much fuel in one week. Appx 60 gals.
I went over and looked at the situation. The furnace filter was missing, and was sucking air thru the open housing. I duct taped that shut pending a filter tomorrow.
It seemed to me that the "hot air" coming out of the duct 4' from the furnace wasn't all that hot. What is the output air temp supposed to be on this or similar units?
The home itself isn't very drafty-leaky, so I'm looking at furnace performance. 10 gals of LP/day is alot for a 1000' s/f home with the 'stat set at 65.
Any Advice?
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