The fines and such are primarily for commercial vehicles, not RV's.
If you are really worried about bridge weight limits, you can get a database of questionable bridges - I think there is a POI for your PND (personal navigation device) for this. There are also truck and RV oriented PND's that will route you around low overpass or weight limited bridges.
That bridge database is on the net as it is a government agency that has ratings for various components of all bridges in the US (deck, structure, ...). I used it to make my own POI and it is interesting to see what is in it and why.
Generally, it isn't too big a deal. The safety margins are usually fairly wide.