There is no perfect answer. I normally keep my vehicles till they drop. I have 170,000 mile on my 2001 Prius.
But for RV vans your needs can change. Started with a 1973 Dodge B-300 van I converted myself while my kids were young. They grew up and went off to college. In 1999 with over 130,000 miles, I gave it to my neighbor at the time who knew a farm family in West Virginia that could put it to good use.
Then I got a 1999 Ford E-350 to tow a vintage Airstream trailer. Sold the trailer in 2005 because my work schedule prevented me from using it much. Sold the van in 2012 with 96,000 miles to get my Airstream B-van. Now after over five years and 100,000 miles I'm planning to down size a bit into a shorter B-van. It will fit better with my changing life style.
2013 Airstream Interstate Lounge EXT on 2012 Sprinter 3500 170Ext
Formerly: 1973 Dodge B300 DIY pop-top conversion