Thanks for all the replies so quickly. The answers were about what I was guessing at (4 to 8mpg) depending on terrain and speed.
For the one questioner, It is a P-30 Chevy chassis and the engine, cooling system and tranny was rebuilt in 2014 and a SWAG on the mileage on the engine is about or less than 6k miles. (The odometer doesn't work any more, Something else to fix.)
I came into possession of this thing through a fluke and I'm trying to figure out if it's worth keeping and throwing some money into or just donating it to some charity and taxing the tax deduction.
The previous owners seemed to take very good care of it, but again it's 24 years old. I have ALL the maintenance and repair manuals for just about every thing in it. And I'm mechanically inclined, but I'm 75 years old and wondering just how much to try to bite off with this project.
Frank