You do not want to rely on a POI to warn you when it is TOO LATE that you are about to hit an overpass. The only way to go is a GPS that has built in RV or truck routing. As has been mentioned, the Rand McNally is becoming more and more the favorite between the two companies that make such a GPS - Rand McNally and Magellan. Garmin makes a trucker's GPS, though it is not easy to find at general retailers that sell Garmin. You want to be routed from the start to avoid all low clearances, weight restrictions, and road restrictions - and if you change routes mid-way, you want it to automatically re-route you on roads that you can travel on. All the POI will do is put an icon on the screen and with Garmin and most units this has to be done in a special manner so that it actually comes up - but you are likely to be on the road and heading for it before it ever shows up!
One thing I highly recommend is if you have Google Earth, is to down load the files off LowClearance's for it. Then get directions from Google Earth before you go so you can see if there is a problem with low bridges/underpasses. These are the same files loaded on Garmin's trucker GPS's.