You in my opinion will need to drive a lot to keep your battery charged off your alternator even for your fridge. Using a lead acid battery, you can only use half the battery capacity vs the lithium where you can use almost 100% with few issues. Every time you go below that you damage your battery. I’d consider a small portable solar panel for those days your not traveling unless you can use some shore power with a battery charger. I have a newer fridge similar to what you got and with 200 amp hours of lithium, two days was stretching it if I didn’t have enough sun. Albeit I had only a 100 watt panel. But a stock alternator would in my opinion have to run a lot, at least without a dc to dc charger. You may find you’ll be replacing alternators a lot if you depend solely on it. I e gone through six in the past 400,000 miles. I was on the road hauling boats coast to coast. No fridge but during my ten hours I couldn’t drive I was running fans and charging electronics draining on the batteries. I assume it’s a decent size alternator to keep two batteries charged all the time. But if I added a house battery, it would seem that would be that much hard on my alternator. Campers have been set up like what you have forever. But that wasn’t meant for full time use. Again my opinion and thought process. I upgraded to a bigger fridge. I had to go with 400 amp hours of lithium to get through two days of heavy clouds. Back to your microwave thoughts... even if you had a 1500 watt pure sine wave inverter you’d have had a difficult time recovering your one battery even without the fridge if you used it much at all. So probably best you chucked that idea. It would be tough also I would think if you didn’t have a good way of monitoring your battery to maybe turn the fridge lower when the battery needs to get through a time when charging isn’t quite making it. Of course, you may have a big old semi with a high output alternator... so there would be that. Haha. Those 12 volt compressor fridges are game changers though. That’s a great deal... but another thing. Mine will not run good under 12.5 volts. The compressor will turn on and off constantly and will eventually fry the control board.