Batteries are an important part of RVing - especially if you plan on boondocking, and no one ever complains about not running out of battery power while boondocking. Yeah, asking a dealer to install a second battery without specifying its parameters, almost guarantees they'll go as cheap as possible.
A second battery is a great idea, but it should have the same amp hour capacity as the first battery (or very close to it.) If your first battery is a standard lead/acid battery, so should be the second one. And it's a good idea to ensure that both batteries are near the same age. None of those are hard and fast rules, but they will help you get the most out of your battery bank.
Kev