If you are just going to be boondocking periodically or aren’t even sure if you are going to like boondocking (or want to get started as budget-minded as possible), I’d go simple for now:
– plan on a small generator. Either a 2000/2200 watt generator that will give you 120v power but won’t run A/C or a 3500 watt generator that will run the A/C.
– replace your 1 battery with 2. I might not even go for lithium right out of the gate if you aren’t sure if you’re going to be boondocking often. If you do go lithium, the cheapest but decent quality route are the Chins 100-200 amp hour lithium batteries. You can charge with your existing converter, but you will only charge the lithium batteries to about 80-90%. Lithium doesn’t care if it’s not fully charged.
– do spend the money on a Victron SmartShunt or BMV so that you can track and properly monitor your battery state of charge.
– get a small 800-1,000 watt inverter. Get adapters to plug the whole camper directly into the inverter (aka 50amp to 30amp to 15amp); you’ll want these anyway. Before plugging in, turn OFF the breakers for all of the things in the RV except for the one or two breakers that make up the majority of your outlets.