For what it's worth (probably not much), the design of our Itasca Class C is such that the 2 coach batteries get directly connected to the engine starting battery (all 3 are hooked up in parallel with each other) whenever we're driving. The ignition key automatically switches ON a solenoid that connects these 3 batteries together whenever the engine is running.
The coach batteries are a couple of 12 volt 100 amp hour deep cycle AGM batteries that will be 8 years old this fall. The engine battery is a regular wet cell engine starting battery that is probably around 5 years old.
As far as we can tell on RV trips all 3 batteries are still functioning fine. When camped and in storage, the RV's stock 13.8 volt converter keeps the coach batteries charged. When in storage a small multi-stage BatteryMinder is continuously hooked up to the engine battery. Of course whenever we're on the road, the engine alternator's voltage (varies from as low as around 13 volts in hot weather up to as high as around 14.5 volts in cold weather) gets applied to all 3 batteries.
Our situation seems to show that under certain conditions, AGM deep cycle batteries can be mixed with a wet cell engine starting battery.