Do you know if the low voltage issue is due to the engine/alternator producing low voltage or if something is discharging the battery at too high a rate?
Typically there is an emergency start connection between the coach'es batteries and the engine battery. Anything from a bad coach battery to a faulty wiring job could potentially cause problems.
For example, if the emergency start connection were mis wired so that the coach batteries were always connected to the engine battery, and if one of the coach batteries were bad, it could discharge the engine battery and/or cause a low voltage condition.
To rule this out, make sure they check the coach batteries too.
I'd ask the dealer to disconnect the emergency start connection - at least temporarily - and see if that fixes things from the engine's perspective. By doing so, you should (and I say should with emphasis) be isolating the coach electrical system from the vehicle electrical system.
Once the systems are isolated, then you should be able to determine if the problem is with the engine or coach electrical systems. And that at least is a start.
Any competent dealer should be able to clamp a DC ammeter to the wiring and see if there are any unexpected currents. Afterall, these guys sometimes charge up to $130 an hour for service rates, so for that kind of coin, you should have the expectation they are competent enough to fix a problem.
Sounds though that they are just throwing parts at the problem. That can become expensive and time consuming, and you may never find the problem.
The only thing that works on a RV is the owner...