You trade for the same reasons you move into a new house, or you purchase a new car:
1) The camper is beginning to nickle-and-dime you to death. Repairs are becoming too numerous and becoming a real drag!
2) You simply do not like the camper any more, for whatever reason? It might be that corner bed. You're sick and tired of climbing over each other (for example).
3) You're board and just want a change, or you want to move to a different style of RV, for example go from a Travel Trailer to a Fifth Wheel or the other way.
4) You outgrow your current camper because of the addition of kids to the family.
5) Your family reduces because kids no longer go camping with you, so you want something smaller.
I think those are the main reasons why people change. As long as you are happy with your current camper, and functionally it is OK, there's no reason to change.... oh ... one more....
6) You're social-lite friend just bought a new one and now you feel you need to get a new one too .... (very bad reason though, but sometimes this is the motivation). You should get what makes YOU happy and what works for YOU, and remember.... you are not trying to impress anyone. Actually, it's none of their business! The camper is for YOU! Not them!
Hope this helps!