Forget the converter upgrade.
For a bit over $200 you can go solar and never have to plug in. Since you BD 95% of the time you really, really need solar anyway. If you have any basic electrical skills you can do the install yourself.
Skills needed: know positive from negative battery side, be able to strip and crimp wires, be able to run wires through a hole, be able to attach brackets to a roof and attach a panel to the bracket.
Two 100 watt panels - $145 shipped from ebay. (or one 200 watt)
one MPPT controller $30-40 shipped.
additional wire and bracket and Lap sealant = $40-50
The MPPT controller should not overcharge the batts, but you still need to check water a few times per year.
You never have to plug in, just keep the panels in the sun (remove snow).
Your batts recharge every day while camping.
Since u live so far north maybe you want to have panels on a hinge so you can tilt them south. This is more complicated but doable.
Keep your existing charger for times you might camp next to an outlet and want more DC power, though you might never need this option unless it is raining, snowing or you park in heavy shade (though panels will still do some charging even under trees).
Lots n lots of topics on solar, you can find everything you need to know just browsing all the threads