Your problem sounds like you either have a bad connection or bad house batteries, but their are several questions that need to be answered. First how many amps were you drawing from the batteries to make them drop from 13.7V to 12.0V in 15 minutes? Second have you serviced your house batteries (cleaned all connections and filled with distilled water)? Third have you load tested your house batteries? With the answers to these questions, we can determine the cause of your problem.
If you have one or more bad house batteries, the question of how many batteries to replace is hard to answer. Normally it is recommended to replace all house batteries at the same time, but if a load test shows that you have only one bad battery, there can be a very strong argument to replace only the bad battery. If you are planning to keep the coach at least 3 or 4 years longer, and the house batteries are four years old now, I would replace all 4 house batteries with a good lead/acid golf cart battery like Trojan.
If you have a need for higher battery capacity like running a CPAP machine at night, or if you are very poor a battery maintenance you may want to consider replacing your house batteries with AGM batteries, but they will cost about twice as much as a good wet cell battery.
Gerald