If stored in a cold Michigan barn all winter without a "tender", you'd be fortunate if your batteries lasted two years. While removing the RV batteries during winter storage is a good idea, not using a tender to keep the batteries charged somewhat defeats the purpose. A discharged battery will still freeze and be rendered pretty much worthless.
I'd recommend buying the Stanley 25 amp multi charge rate battery charger:
https://www.stanleytools.com/product...e-start/bc25bs
One for the features of this charger is the ability to equalize or recondition the battery and help reverse sulfate build-up on battery plates to improve charging carrying capability and extend battery life. This charger may be able to bring your batteries back to serviceable levels.
Even if you have to buy new batteries this charger is a must for properly storing and keeping batteries well maintained.