Do you know if the original Univolt is still in the coach? If so, I would recommend upgrading to a modern "intelligent" converter. Before you decide on a battery.
Depending on your planned use of the coach. How often and for how long.
If you are going have the unit parked where power is available and have a modern converter. You will not need a battery right away. All DC systems will work fine, as long as you have shore power.
You will however need a battery in the coach while towing in order to power the breakaway switch for the brake system.
As for batteries. It is an individual choice.
Go for the big bucks and buy AGM or similar battery(s)
Or go to Wal-mart and buy a RV/Marine battery(s).
I choose the latter. We have the coach out multiple times over the spring, summer and fall and even sometimes in the winter. I am going on the 4th year with a Group 24 battery that cost around $80.00.