It's tougher to refinance. We did it a couple of years ago on a 4-5 yr old unit- went from over 8% to under 4%. But they had to tack in a little as we said we were going to upgrade to residential fridge. But they wouldn't go 5-figures.
If you buy a new one, unless you get a very good deal, it might make your payments higher and still won't pay off the 401k loan.
We borrowed from the 401k to pay off the balance of the F450. So we didn't end up increasing the budget. Rather we're paying ourselves interest instead of Ford Credit.