I would sell it.
You will find that the sales value drops a lot once it is 15 years old, and not able to be in many campgrounds with a age limit. While the roof is still only 8 years old, in a year or two, you probably should consider coating it with a layer of EPDM if you did keep it. This will prevent leaks, and give the coating a solid surface to adhear to. Mine, I waited until it was about 12 years old, and wish I Had done it sooner.
I think you will find that the replacement fifth wheel will have a larger fresh water tank, better capacity of the grey and black water, so dry camping in it would be more comfortable. Why store and maintain two trailers when you can only camp in one at a time?
I could understand if you have a old beater trailer for use as a hunting cabin, they tend to smell like the deer that you bring home inside it, and mom will hate camping in the new fifth wheel if it smells like that! But I would not invest more than about $2,500 in a hunting trailer.
It sort of reminds me of the 4 year old van that I used to have. When I bought a new Honda CRV to tow behind the RV, the wife wanted to drive that car daily, not the older one. I think you will enjoy camping in the fifth wheel a lot more than the travel trailer.
Good luck!
Fred.