If your concern is that a Li battery lifespan is reduced slightly from being kept absolutely full, 100% of the time, then the solution is easy.
Just keep a small load on all of the time. Example, LED light, refrigerator, fan, inverter, etc.
The solar might fully charge the battery, but it won't stay that way for more than a few hours per day.
There is nothing wrong with having a small amount of cycling going on with your power system - my theory is that once you turn on a power system in a vehicle - just keep it on running normally all of the time. That way you will know that it will work when you really need it.