Your bike can do 105 MPH (maybe not with the windshield and saddlebags) or get 40+ MPG. Not both at the same time. Not much better MPG with those Dunlop tires. Your MPG is low but not ridiculously so. Check out this thread
to learn more. (Lots of good stuff buried in there.)
Since you're running into troubles at 85 MPH, you know you've got an issue or two. My guess? You've got several off-spec conditions that work together to rob you of power when you get high enough RPMs. The bike is new to you so, do you know when
- the spark plugs were last changed?
- oil and filter?
- air filter? (Don't care how it looks, care how it performs.)
- fuel filter?
- final drive oil?
- air/fuel mixture was last checked and/or adjusted?
- carbs synched?
The total mileage on that bike says every one of those items* should have been done either in the last 3,600 miles or the next 4,400 miles. Coolant should be replaced next year.
While you're attending to any that have been missed (I'd definitely concentrate on spark plugs, air/fuel mixture and carb synch. I'd change the air filter, too -- but I live 100 ft from a dirt road and have to use it to ride anywhere.), check and/or replace the brake fluid. Lube all the control cables, including the speedometer cable. Check the brake pads.
You've got stock exhaust pipes. I would not expect that the carb jets have been changed. There is not yet any reason to tear apart the carbs to check. Your shocks are stock, too. Probably your fork springs. You can dramatically improve ride and handling by changing both -- although new fork springs might make the front end sometimes feel a little squirrely with those tires.
* Except the fuel filter -- it's not on the maintenance schedule. Should be.