Voltage on the battery while charging from engine or generator can be revealing. It needs to be over 14 volts for effective charging.
The battery voltage immediately after charging will still be the charging voltage. It takes hours before it goes down to be of some use estimating the state of charge. I disconnected the house and engine batteries on our class C in September and they are still more than 12.6 volts. Solar is the answer but get at least 100 watt panel.
A battery monitor displaying the % of full charge accurately by means of counting amp hours in and out can be had for $30 on eBay.