I would never ever trust a gps system or GS system to warn me of low clearances. The only AUTHORITY on this in the US is the Motor Carriers Atlas and mileage guide.
Having said that the chances of you running into a low clearance west of the Misssissippi is nil. Non existant if you stick to the interstates. They do exist but most have been redone.
In downtown areas of Chicago, Boston, NYC and some other eastern cities low clearance bridges are much more common. But who takes their TT or MH into downtown Chicago or NY?
You can go anywhere a commercial carrier can go with a 13'6 trailer.
But if you want to really know the low clearance areas stick to the Motor Carriers atlas.