My 10,000 lb boat with a TAMD-41P (newer version) burns 2 GPH at cruise (2000 rpm's, 7.5 knots) FWIW...
Fill your tanks, run for 40-50 hours and tank up again for a good average for you.
My rate is measured over 4 years and 300 hours..
With your larger boat, I'd expect 3-5 GPH..
At a 2K rpm cruise they are pretty economical assuming you are propped correctly..