A diode may fail and cause your predicament but I doubt it.
What you need to diagnose is what causes the draw from the batteries. This is usually done by process of elimination and the use of an amp meter in series with the battery connections.
Do you have a 12 volt distribution panel? How is your 12v wiring configured? What make and model is your rig? Do you have a converter or other battery charger?
You can do some diagnostic work and then remove one of the terminations on the batteries to forestall loss of the new batteries. If you have a small automotive type circuit breaker that is mounted to frame ground, that would be one of the first places to check.