Garmin and Rand McNally both sell GPSs for RVers, in which you can put the dimensions and weight of your RV (toad, too). You can also get POI add-ons for some GPSs that tell you about a low bridge, etc. That being said, their greatest accuracy and completeness will be on state and federal highways (there might be something missed), but on other roads they're less likely to have complete data. A paper copy of the Trucker's Atlas can also help you find areas to avoid.
But in the end, you must still maintain vigilance for spots missed by the above -- road signs are SUPPOSED to be posted for clearance and weight problems, but there's never a guarantee. However that Mark One Calibrated Eyeball can be used (after calibration, of course), and is a vital tool for avoiding low/tight spots in gas stations, parking lots, city streets and other locations, even the areas the electronics are supposed to help.
And if you get to a spot you question, in spite of the above help, don't hesitate to get out and look more closely before entering the area -- it's cheaper that way.