re: "80AMP Portable Solar Panel
" -- you mean 80 watt?
Battery capacity is a matter of weight although AGM batteries tend to be on the low capacity end of the curve. Generally, wet cells tend to be most cost effective and whether you use 6v or 12v
is of no matter. It is total battery bank weight that counts.
As for monitoring the battery, voltage will tell you state of charge. As with any measure, it must be done properly and properly interpreted. Generally, don't let the voltage go below 12.2v as measured after no significant charge or discharge for at least a half hour.
re: "integrates current over time to give an accurate idea of the state of charge.
" -- As for the electron counters - they are a lot of fun but do be aware that they must be properly programmed and even then don't measure the influence of such things as age, cycle to cycle variations, and other significant factors. (temperature, usually, use profile, somewhat).
An 80 watt panel is minimal for 80 pounds of battery (like 2 group 24's). Plan on about 2 hours at peak output for your solar energy harvest per day which would be 160 watt hours for that panel. Keep in mind that the typical residual load in the trailer is about 10 watts which is about 240 watt hours per day.
so much hype ... so little time! ;-)