Well Sir, this truly depends. Sometimes there are slight differences in the two sister (Winne and Itasca) manufacturers. One can ASSUME that the house battery tray MAY accommodate more house batteries than originally equipped with. But, I've had two Journey owners that have had to modify their battery house battery tray to accept the change over from the traditional (2) 12V or (3) 12V deep cycle Marine/RV batteries to, (4) 6V Golf Cart deep cycle ones.
First, either get on line or, contact the seller and determine just what is in the house battery tray at present. After all, like you, it is possible that the present or, even a previous owner may have altered the tray and or battery arrangement already.
When we purchased our '04 Itasca Horizon, two slide 36GD with the 330 CAT, about 11.5 months ago, it came with (3) 12V Interstated SRM 429 12V so called "Deep cycle" marine/rv batteries. Long story short, I had two go bad, and all three were only 1 year old at the time we purchased the rig. So, I contacted a friend who's got one of the Journeys and who's already done the conversion to (4) 6V G/C batteries and asked about his change over. He stated he had to remove the tray, cut it and weld in sections to make the tray large enough to accommodate the new group of batteries.
Well, I was prepared to do that when I went out with my trusty tape measure and checks ALL the dimensions,,,, twice....three times. Then, off to Costco I went and checked there G/C batteries because they had the best price on the planet at the time. Well, as it turned out, I did not have to modify my tray, as all four of the batteries would fit with room (not much) to spare.
So, after talking to a few other folks with Journeys and Itascas of the same year vintage, it appears there are differences in the trays. Why, who the heck knows.
I can tell you, changing from the (3) 12V units to the (4) 6V units is the best move I (and others here) could make. Take a look and see what you think. If you'd like more info on how I did it and what was involved, PM me and I'd be glad to help.
P.S. The first pic is the original arrangement with (3) 12 V, the rest of the pics are of the newer setup with (4) new Costco 6V units