Tell her by the time you get done paying for the new truck in 4-5 years your Range Rover will still be worth more than the truck that is 10 years newer.
What about full coverage insurance.
That will cost as much as the car payment. Double up on funds for that.
Make a list or print all the good clean intelligent input from those of us who are adult car owners and have it ready for ammo during the next discussion.
I have owned 37 different new, used, exotic and rare cars and trucks and MY 94 RRC LWB runs and rides better at 169,000 miles than most of the $30K-$40K vehicles that I bought NEW!
The milage will be almost identical in the two vehicles in town so that point is out the window.
You simply tell her. I feel safer in this vehicle, I don't have to worry about dents, dings and scratches, it won't depreciate any more, it's paid for, I'm getting to know it well mechanically, I trust it to get from A-B and just think if all of those factory parts made it to 160,000 before you replace them; they should be good for at least another 100,000 miles after they are replaced. These things are tough, have alluminium bodies and won't rust if you wipe them down.
Every dollar you put into your Rover will come back to you.
Every dollar you put into a newer vehicle will evaporate in the next 5 years!
Good Luck!

Oh ya, Tell her that this is also a rare vehicle that will eventually increase in value.
There were only about 5700 of the 94 RRC County LWB imported to the U.S. that year.
$800 a year is only $66.66 a month to drive it.