The breaking of strings in RV shades is a very common occurrence. Before we replaced the day/night shades in the motorhome we had previously with MCD shades, I used to buy the string in 300' spools.
A big reason for strings breaking is sharp edges on the metal tops and bottoms of the shades. The Dirty Blind Man
has repair kits and parts for RV blinds. One of the things contained in the repair kits (and also available separately) are nylon bushings to put into the holes in the metal strips so the string doesn't run over the sharp edges.