Simplest battery monitor and shunt? Advice for non-expert

It is a dilemma. In order to accurately measure what is "left" in the battery, one needs to know the capacity. How many folks have actually performed a test to determine the number of amp-hours available?

But, even if one did, capacity is affected by temperature, age, demand and other factors.

I solved the issue by having a bank that was about the size of the original Tesla power wall, and solar to maintain and recharge between trips.