If you're just trying to boondock for 3 days I would suggest getting a larger group 31 battery. You can get a good battery for <$150. Use that as your main and swap over to your smaller one if needed. Charge them both up before you leave the house and again when you return.
Carrying a second battery is much easier for a short trip than hassling with a genny, and cheaper.
Get a baseline data point by driveway camping. Use a volt meter to monitor your battery as you go. Don't rely in the idiot lights on your panel.
If you are determined with buying a genny for boondocking just to charge the battery any inverter type will work fine.