If the current batteries are over a year old it is normally recommended that you do not add new batteries into the bank but install all new.
Unless your previous coach had two batteries and you know how that fits your usage I would wait before adding/replacing until you see haw the current setup works for you.
Paul, Kathy, and Tux the Mini Schnauzer
2014 Tiffin Phaeton 42 LH, 2013 Honda CRV
"When the time comes to look back, make sure you'll like what you see"