I did that with 911.Didn’t notice the output voltage. Between that and the dry plates, put your local FD on speed dial.
Do you have any picture of the battery labels? Hard to guess at the capacity from the top. It looks like it could be a group 24, or maybe group 27. In the best case those would be 80 or 90 amp hours respectively. But it’s hard to say whether they’re true deep cycle or less durable starting batteries. There’s also no way of knowing by looking if they’re anywhere near their original capacity or if that has decreased from age and use. Only way to know the accurate capacity would be a 20 hour load test.P.S. How much capasity of this batteries any guesses?