1983 GMC Value Van 3500 6.2 L V8 fuel question

[elbowroom]

Hi, I have the above van and I am looking to find out what the fuel capictyis and the average miles per gallon. Does anyone out there know this information or where I can find it?
Thank you.

[AVM62]

33 Gallon fuel tank. I do 10-11 mpg 1983 GMC Vandura 3500 6.2L Diesel 23' Motorhome.

[elbowroom]

Thank you very much!