The full picture is slowly emerging. Finally, after 4 pages :)
You are a very light user, you have an adequate inverter and more-less adequate solar (have some doubts on 120W being adequate, but in sunny climates - maybe).
Yes, you can get away without using a 2nd battery.
But I don't see any benefits in keeping 2nd one as a spare, as opposed to using it. Here is why:
- Batteries rarely die suddenly. Usually, they wear out. It takes YEARS. You'll have plenty of warning.
- Your solar will (probably) keep 2 batteries same happy as it does with one, because again, you're a light user - probably starting with 85% full in the morning, so your solar is capable of generating more amps than 2 batteries would need, because batteries are near full. When they are near full, they can only accept very low charging current. (This would need further testing, re: your weather pattern etc, but you've got the general idea).
- Your spare battery (if you keep it spare) will self-discharge slowly while you're not using it (though this won't be notable if you camp only 3-4 days at a time).
As to the inverter - I like SMK idea to mount it near DC fuse panel and converter, 'cause you'll find there DC lugs and a convenient spot to wire the AC from inverter into your AC circuit. Just make sure you have access to inverter switch, because for a very light user with very small solar the inverter should not be left On all the time, wasting precious AH.
