That is not out of the realm of possibility. Many modern MH can get 8 MPG or better. It depends on the motor, year, weight, gearing, etc.
Here's an idea. Load it up, fill it with gas and go on a weekend camping trip (say 100 miles away). When you get back, refill with gas, and devide the number of miles you traveled by the the number of gallons of gas you used. Then you would know for sure