I added a 2nd battery when I purchased my new Minnie Winnie 24" 22R... single battery under steps, room to add 2nd... took pic of battery which camp with the rig... Napa battery... went to Napa showed them the pic... $92 later installed myself in less than 15 min... all is good... will be adding solar in the fall.
There is only vinyl in my rig, but I added carpet myself which covers up the steps coming in, it is not permanent and can be removed in order to check battery water levels and also there is venting thru the flooring under the batteries so as not to vent into the rig.
Been full timing since Aug 2015... Enjoy the process getting ready and just love life on the road.