There are many reliable SoC battery meters.
Yes Bogart's Trimetric is a good one.
On FLA hydrometer works well.
Measuring amps dropping at tail end of a full charge will also at least tell you Full.
I agree with using something like the Trimetric. I use one and it not only measures the electricity being used it also tracks the incoming charge as well. If you were just using your van for weekend trips it might not be worth the money. But if you are full timing or close to it then something like the Trimetric is an amazing tool to have. You configure it to your batteries Ah size and it will track all of your usage. It can then give you a very accurate state of charge display in percentages. After running my fridge and Maxxfan all night I can look in the morning and see that I am around 90% SOC.
With an AGM you can't use a hydrometer and I don't think just glancing at current voltage will really tell you accurately what the SOC is. And with something like the Trimetric battery monitor it is super easy to just look at the monitor and know where you are at.
As to the question of can your battery fully charge while using solar and while your lights and appliances are running. I think it is more of a simple equation of electricity coming in versus electricity going out. If you have 10 amps coming in and 8 amps going out your battery will accept the leftover 2 amps and keep charging. If you have 8 amps coming in and 10 amps going out then your battery will be losing the 2 amps per hour.
My battery might start out at 85-90 % SOC in the morning because it was using electricity all night but by noon I could be back to 100% and stay there all day because I have more then enough coming in to run everything and still have plenty to recharge the batteries.
