The BCI case size and CCA rating is usually on the side or top of the battery. That's all the info you need to buy any brand. According to Freightliner chassis specs, the Discovery was originally equipped with 760 CCA Denso batteries. Any starting battery with a 760 CCA or larger rating will work fine. Freightliners typically come with BCI Group 31 size batteries, but the 760 CCA size could be the smaller Group 27 as well.
Interstate offers a Group 27 with 800 CCA and a Group 31's with ratings of 700, 950 or 1000 CCA
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC