Something like what you're considering would probably be the simplest solution. The one obvious drawback is that you can't charge the coach batteries from the engine, as you pointed out, and if they're low the alternator would provide a lot more charge than the solar panels alone. One solution there is to replace the BIM with a plain old continuous duty solenoid wired to the engine run circuit (and maybe also to the emergency start switch if you have one) such that they connect when the engine is running. This is a simpler setup that many RVs use by design.
The engine computer and some related bits are mainly what would consume power from the engine battery...that, and self-discharge. If you're worried about the engine battery running down during storage, you can either get a small solar panel to maintain it, or if on shore power just use a little battery maintainer charger of some sort. If you have a lighter socket connected to the engine battery that's live all the time, that may be a convenient place to plug in a maintainer charger.
