fuel pump poorly tightened or gasket defective (they are famous for this....i have these if this is the cause & you are unable to locate)....although from the description above this sounds like an excessive losss. have you place a piece of cardboard on the ground to help determine location/locations?...any blue smoke from exhaust...what's the exhaust tip look like?....what do the plugs look like?

-never travel further than a case of oil