Most people buy and sell on an emotional basis, and so are behind the curve, so-to-speak. Sell your truck now, and you will take the lowest value. Then later on, when you will discover that you really did need a real truck, you will over-pay, along with everyione else who will come back to a truck after the panic is over.
Now, if the truck only hauls you around, then it is another issue.
Figure your cost per mile--fuel, maintenance, insurance, and loans.
Then figure the same for a new truck. Show her the paper--it will probably not matter--she has made the emotional "decision", and facts will not sway her. But the exercise may help you to realize what the cost really is.
If your old truck is paid for, it is hard to imagine how a new truck can be cheaper.
Gas isn't cheap either.