I had a HitchHiker 5er, ~15k lb, that I put IS on 6-7yrs ago. It made a huge difference in smooth towing. At the time i oversized the brakes but didn’t go for disc.
Three years ago we bought a new Vilano, ~16k lb 5er. Given the history of VanLeigh running gear issues we’d decided to replace with IS within the first year of ownership. Very happy we did for ride, AND peace of mind. The IS suspension is not only more robust than traditional springs, but if it were to fail the failure mode is much more graceful. Absolutely an IS fan.
On our Vilano we also elected to put on Kodiak disc brakes at MORryde. HUGE, noticeable difference. Prior to that, and with HH with oversized brakes, I would have at least one or two stressful stops each year where someone cut you off, of light change issue where someone stopped short, etc. With drum brakes these we’re always standing on the peddle and *hoping* I’d stop in time. With disc brakes, I just stop! Best I can explain it would be that disc brakes versus drums for stopping are what diesel is versus gas for pulling. They just put a smile on your face and give you confidence.
Only you can decide if it’s worth it for you. But the fact you intend to keep your rig for a while, and have the $ to put Lithium batteries in, suggest that MORryde IS and disc brakes would be a worthwhile investment for you. Only bad part right now is that I’ve heard MORryde is booked out a year or so.
Good luck with your decision, and safe travels.