Batteries should be replaced in sets. From a battery manufacturer's owner's manual is the following quote: "Always use batteries of identical make, model and with the same manufaturing date." That quote is from a section relating to multiple batteries.

Since you are confirming the batteries are in series, I would look at the equalizer as being part of this potentially. I am not familiar with how Newell is set up, but speculating here, if one is 13 volts and the other 2 volts the equalizer may not be doing its job. When you maesure across the 12 volt and 24 volt posts on the equalizer the 12 volt side should be 1/2 of the 24 volt side within 1/2 volt. There is more detailed information in the Vanner owner's manual but that is a quick check. You do have an equalizer, correct?