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Not having a trailer level could impact mileage a little but there are other possible worse problems to think about. A sag in the rear end causes your headlights to seem like highbeams to oncoming traffic. Too much sag means you have no vertical play (your truck rides on the bumper stops) and this could make for uncomfortable long rides and no telling what it does to the rear suspension in the long term. Also, if there is any significant downward angle on your 5er, I have read that is worse than an upward angle and could impact the RV suspension parts and wheels, but I don't know how much angle is of concern.
Finally, you might weigh your rig combination at the axles and confirm against the gross weight ratings on your TV and RV to make sure you exceed NONE of them. Then the most effective fix is air bags since they allow convenience when towing or not.
Your gas mileage seems to be very similar to other truck mileages (all brands) so I don't think it's that bad (just think if you had a gas engine?). 13 MPG is outstanding for a consistent 5er tow. The expectations are 9-12 for most. I got over 13 MPG but only when I reset the computer while descending the "I-5 Grapevine" and continued in the flatness in the San Joaquin Valley. My typical is 10-12mpgs combined driving depending on terrain (any hills and my mpgs will plummet).
I hope when the kids graduate college and get their careers going, they gift you with airbags!
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