Gas records.
Are you keeping accurate gas records?
Since I bought my rig, I have logged in every gas purchase and the odometer readings. I always mark my purchase with whether or not I filled the tank full. That way I can go back and calculate the actual gas mileage. I transferred the log to an ongoing spreadsheet and worked out the costs and miles per gallon.
Most folks when estimating their miles per gallon, just get it plain wrong. You really need to keep detailed ongoing records to get an accurate picture. In my log book I also mark down everywhere I have spent the night. So the entire time I've owned my rig, I know where I went, where I camped.
That information has been fun and very helpful, as I sometimes do repeat visits and similar trips to get back to places I enjoyed so much, I wanted to go back again.
Now in my case, I started out keeping up with generator use and gave up on that, too much work. So, my gas mileage does vary because the generator drains off the same tank. So that throws a monkey wrench in the works. I have an hour meter on my generator, but I am not convinced it works correctly.
So the summers I workamped, my miles per gallon was much higher than the summers I traveled. This is because when I travel in the summer, I sometimes have to use the generator on hot humid days in addition to the dash AC. When I was workamping all summer, I only used the generator once or twice a month to exercise it.
Driving around mountains made my miles per gallon go down, driving around flat Florida, my miles per gallon goes up again.
Sometimes the gas mileage went haywire during an emergency. For instance I had emergency surgery while my RV and dog were parked in the hospital lot running the generator nonstop. Then it was 5 miles to the campground. So 24 hours of generator plus 5 miles driving reduced the 55 gallon tank about a fourth which is about 13 gallons. Noting that in my log book helps me remember when I get around to compiling my spreadsheet.
It would be easy to think something was bad wrong that between those fuel fills I got such dismal miles per gallon, but I was careening around medical facilities with the generator running full tilt boogie.
Headwinds and tailwinds can affect gas mileage too as well as whether you are towing or not towing.
To budget for trips I figure a worst case scenario with only 6 miles per gallon and factor in the highest cost per gallon for gas purchases. That way I always come in under budget on the trip (for gas) when all is said and done. (Except for the trip that landed me in medical hell.)
I generally get about 8.5 miles per gallon AVERAGE in my Class C including the generator usage (that is after compiling several years of keeping records) and I often do travel with full water tank and full gas tank. I don't like to let my gas get below a half tank. If an emergency comes up, I want gas for the generator. The only time my rig has needed road side assistance was on overly hot humid days, so having that gas available to fire up the generator was life saving.
If you don't have good accurate ongoing records, then it's possible you don't have a problem at all.
By the time you factor in generator, towing, winds, mountains and so on. 5.5 mpg may be about right for your rig.