Check the voltage levels of the individual batteries. They should be within a volt, ideally within a couple tenths.
Within a string that has some age on it, you may eventually develop some weak sisters. They will only accept a lower charge, for whatever reasons. In the same string, the stronger batteries have to make up for the weak ones.
Offhand I don't remember whether it's the strong or the weak batteries that are thirstier but it doesn't really matter -- the thirst develops near the end of the battery's useful lifetime. You are going to need to replace the batteries that won't accept and hold a full charge, and as I remember those are going to be the heavy drinkers.
Not holding a charge is symptomatic of an aging bank or a few bad batteries. You don't have to pull them out of the bank to test individual voltages. A few weak batteries will over time destroy the stronger batteries in the string because they are being overworked.
The battery temp sensor(s) should go somewhere in the middle of the bank, their location isn't critical. I have mine taped onto a piece of plastic so I can pull batteries if needed without the sensors interfering. The sensors are being used to give a ballpark value, not a very precise one -- at 110F the batteries need fewer volts to charge them than at 20F, but the difference between 70F and 73F is negligable and the temp anywhere in the bank (unless it is truly huge) are going to be within a few degrees.
Lordy, I just noticed the date on your post, I have been gone from here for a long time.