You can get the low clearance as well as any subject you could be interested in, just down load to your GPS, free if you do a search. POI-points of interest
http://www.poi-factory.com/node/33128
But to the OP even with all the newest tech you can still find your self in a problem as all must be entered into the system by SOMEONE.
As to a NEW BEST unit, for me it would be either of two Garmin units, one is a trucker unit and one is a RV unit. Both are 7" screens and have all the tools to help avoid over length and over weight and tunnel and propane restrictions. As well as good routing(I don't think there is a GPS with great routing) as one must still be very weary of and look at the route for any ???? points. All that said I wouldn't go without a GPS for ramps, lanes, turns, potty stops and I really like the picture in route that tell you in advance lanes so you have time to move to the correct lane.
This is the unit I will be buying next Garmin Dezl 760LMT it also will interconnect with your cell phone calls and if you wish can use the data from your phone for really up date info.
LEN