I just wanted to thank everyone for their advice on the situation.

As a follow up, I filled the tank again once the needle was dipping into the
red.  I put in 51.107 litres or 13.501 gallons.  That means that there was just
under 9 litres left in the tank when the needle entered the red zone.  I drove
the van approximately 160 miles on the full 16 gallon tank and drove until I
ran out of gas (I kept a spare can in the van so I could make it back to the
gas station).  This means that I'm getting approximately 10 mpg!  This is all
with the van unloaded, only me and a can of gas (propane tank was empty as
well).

I'm going to check the fuel lines for leaks tomorrow when it stops raining.
Also, there are no dents in the fuel tank either (someone suggested that, but
the tank is taking on its full capacity).  Any other ideas are appreciated.

Thanks to everyone for their assistance,
Ryan