I think your Knight will out last 90% of anything new out there as far as 5th wheels go and trailers. My vessel engine sits most of the year and it has 28000 hours on it with normal maintenance. I bought my last motor home with 400 hours on it representing 44,000 miles, and it is 14 years old. Sitting is not a problem with appropriate maintenance; machinery and coach works.
Agree with Dennis, there is practical and then there are heart felt dreams or just plain enjoyment. I don't think there are many owners that have driven the wheels off a good DP. They last a long time.If staying in the RV mode, I think you are miles ahead keeping the unit you have; from a financial and stress related point of view. (I would never trade my Foretravel for a new 5er plus cash, forget it) I would miss the view out the front window for one. If I had to go 5er, I would still want to eventually get back to a class A motor home.
If still in the game, and affordability is a wash, and you can live indefinitely with your Knight, Keep it. Pack up and live in it once in awhile (a week or two at a time) outside of town and enjoy your coach. Speak with other "camper's" face to face and find out how they justify their usage.