The day-night shades in our living room needed to have the cords replaced after just two years. The shades were made by Irvine Shade & Door Inc of Elkhart IN, phone 574-522-1446. I called the company to get replacement cord sets and instructions. They said that RV manufacturers make the cords way too tight which causes them to wear out (fray) quickly. Our shades were definitely strung very tight.
None of the old cords were broken so we restrung the blinds by fusing the ends of the old and new cords together with a small flame (burning a toothpick) and used the old cords to pull the new cords through. The fused area had to be trimmed smooth so it would pull through the small cord guides in the shade. This is quite tricky to do, but possible using fingernail clippers, small scissors, etc. Make sure the fused bond is strong so it doesn't break while you are pulling the new cord through!
With this method only the top cover of the shade needed to be removed. The top of the shade has three drilled holes for the mounting screws. These holes make sharp points in the aluminum which needed to be bent away from the shade so the top cover would slide off easily.
On the 6-line shade, three cords on each side are attached to a spring at the center using metal crimps. I tied a bowline knot to secure the cords to the spring.
Here is a diagram of cords for our 6-line shade which I don't believe the company has an instruction sheet for.
Also - be sure the strings you are sent are long enough! For some reason, some of the strings for our 6 string shade were short, so we had to redo them. The manufacturer actually send us two sets of four strings for that shade, so fortunately we had an extra couple of strings to work with.
We restrung one four-string shade and one six-string shade. Another shade we had replaced before we knew about the restringing option. All had badly frayed strings. We loosened the strings on the remaining large shade.
We keep our shades strung very loosely now so that the cords won't fray. This means you have to pull the shades all the way to the top to secure them well before moving the coach otherwise they tend to flap around when driving and make noise.