Had the IS and disc brakes installed on our 2018 KPTS the in 2018. In my opinion it is the best investment one can make for their 5er. The difference in the ride for the trailer is phenomenal. Prior to having the IS installed the trailer would bounce 5-6 times after going over a freeway overpass. After the IS the trailer may bounce 2-3 times and the shock is far less than before. Also had the IS installed on our 2001 5th wheel after the chair in the back flipped during transit on the freeway. After having the IS installed on that 5er we went back across the same stretch of road and the chair never moved. I have owned three 5th wheels since 2001 and I am a firm believer in the IS system and am of the opinion that the more you can do to smooth out the ride of the trailer to reduce the shock on the trailer, the trailer contents, and the tires, the better it is for the unit in the long run.
I would also add that the hitch in the tow vehicle plays a big part in the ride of the trailer. Although expensive and bulky, the Hensley Trailer Saver also makes a huge difference.