Welcome to RVF, @Provinge, glad to have you aboard.
I think we need a little more information to be able to help you with your furnace. First, is it supposed to run on "batteries only," or are you wanting to modify it to do so.
Then, there are so many RV types and models out there that it is probably unlikely that just the year and make/model name will be enough information for us to go on. And, as @Neal said, posting your question in one of the more specific forums within RVF will probably get more attention.
I greatly appreciate your comments and responses regarding my question (@TJ&LadyDi and @Neal). I'm completely ignorant regarding what other specific forums I would post this question to. Maybe I'm mistaken, I thought I had posted it to a Fleetwood owners forum. We have a Providence model year 2008 40x. We've never taken it out of the storage unit and are unaware of the furnace can run on batteries (from the factory). Sorry for the confusion, likely why I've never used discussion forums in the past.
OK...I think we may be headed in the right direction. Since you posted in the Fleetwood Owners Club regarding a "2008 Providence" and subsequently added "40X," I think we are talking about a motorhome. A quick online search shows that Fleetwood produced a Providence 40X, so I think we have your coach pinned down to that year/make/model. In case you don't have a manual for your coach, you can find one here...
Unfortunately, as with most RV "owners manuals," it doesn't provide much specific information about actual operation of the furnace. So, another online search for "2008 Fleetwood Providence 40X furnace operation" produced a bit more information. It would appear from what I found (and, I did not check all of the search results), your furnace uses a 12V fan/blower so should operate from either 120VAC shore power or 12VDC battery power. The caveat is that furnace fans/blowers from this era eat up battery power pretty quickly. Below is a link to a post discussing this...
One other caution here regards the condition of the coach batteries. It sounds like the coach has been in storage for some period of time. If it was not plugged in and keeping the batteries charged, they may have been damaged. You probably should "load test" your coach batteries to determine their current condition. Obviously, bad batteries will not operate the furnace very long.
You may want to follow up with an online search for further information about the furnace operation in your specific year/make/model coach.
Thanks, @NWIP, but I don't know that it was THAT good.
I do agree, @Provinge, with the thought that you shouldn't give up on posting to RVF. It is a learning curve for all of us, but there is a huge amount of information that resides here. I learn things here daily. Keep asking questions and posting comments. This is a great (and, very helpful) RV online community.