Even though the fill line is bigger, it's not threaded and attached to tue fill hose. It will come back out the overflow as well as the fill hose opening when full without pressuring the tank any significant amount. Don't open the filling faucet all the way if you are that worried about it.
I would say a 1/2 tank sloshing around going down the road puts more stress on the mounts than a full tank. Ask a truck driver driving a tanker what a 1/2 full tank does to the truck when the wave hits at a start or a stop.
Most campground I go to either have no water or the water is so hard to access you would be blocking the road for a 1/2 hour trying to fill your tanks. Very few have dump stations with water. Maybe that's a northern climate thing, those systems all eventually fail from freezing and cost too much to replace. Now they hardly bother to put in fresh water.
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