My HW27S/C had a standard group 24 automotive size battery. Most dealers install a dual purpose Marine battery, though mine had a general Interstate brand auto battery. They are not the best for camper use since they are really designed for car and truck starting.
The battery box will be easily replaceable with the next size up if you should choose. I would suggest a true deep cycle battery. Trojan makes some very good ones and occasionally people post up good deals from places like Sams, Costco and Meiners. Also most brands like Interstate will make a true deep cycle for fishing boats that are almost always cheaper though their retailer as opposed to Camping World. Occasionally Bass Pro offers their house brand XPS on a good sale as well.
2013 Coachmen Freedom Express 320BHDS pulled by a 2005 F250 King Ranch CC
A rainy day camping is better than a sunny day at work.