I've talked to a lot of full-timers over the years before we started full-timing, and in the time since.
If the couple/ person were not experienced long time RVers - they tended to purchase a second full-time rig within two-three years of starting to full-time. Didn't get it right the first time. (Applies to us - we have realized we would be better off in a 33-34 ft gas class A).
Once folks get a rig they like and that works for them - they tend to keep it for many years.
Full-timers seem to prefer to keep their rig 6-10 years. The number of full-timers I've met with over 10 year old rigs is significant, but most would like to get a newer rig, but haven't found one which fits their needs and budget.
Experienced full-timers seem to be very comfortable with purchasing a used 3-5 year old rig and keeping it for 10 years.