Larry,
Based on your planned use, lithium seems reasonable. To fully charge the batteries options include replacing your converter with with a stand alone lithium compatible converter, adding an inverter/charger or solar with a charge controller set to lithium.
The inverter/charger would give you a/c power while boondocking plus give you a charger that can be set for lithium.
If you don't change anything other than the batteries you will still have more capability than you currently have.
There are several threads here that have covered these questions. Here is a fairly recent one:
https://www.montanaowners.com/forums...ad.php?t=79933