Have you setup your vehicle information (weight, height, etc)?
That "should" help.
But I will tell you that the algorithm the Garmin has been using this last few years for trip routing has some serious flaws: not just in routing your RV where it shouldn't go, but just in routing altogether.
Since Lowrance gave up on the auto GPS market I have used Garmin GPS in all my vehicles and have been very happy with them, even more so after I bought a Magellan combo GPS/dashcam unit on sale and discovered what a piece of crap that was.
So when I got our MH I purchased a Garmin RV660 GPS. It continually routes me out of the way unnecessarily extending my travel time for no reason. It seems to get worse every time it's updated.
This year I needed a new GPS for my daily driver as the old Garmin in it refused to auto zoom correctly anymore: If you were traveling fast it zoomed in to street level and if you were traveling slow it would zoom out so you couldn't even see the road you were traveling on. So I got a new Garmin Drive 5" LM EX GPS. It suffers from the same poor routing problem.
To give you a simple example:
This is the Clearwater/Largo area near Tampa. Driving east on Ulmerton road the GPS will take you off the main road at the intersection onto the ramp and then back onto Ulmerton road instead on just going straight on Ulmerton road.
How crazy is that?
Bottom line is: The GPS is just one input into your route planning, don't follow it blindly.