I hear you on the voltage drop; however, consider this - if your charging system is healthy, it will put out around 14.5 volts. The best batteries only charge to 12.8. For that reason, the minor voltage drop across the isolator isn't enough to prevent fully charging your battery (good isolators work in the 10th of an amp range)

Also, by using a deep cycle battery, you have more use of your capacity because you can more deeply discharge it without damage.

 

Only Schottky Battery Isolators have low volt drop. Your common ones will have a significant volt drop.
The Schottky isolators have a street price of around $180.