Our recommendation remains - - if your house and vehicle batteries are of the same chemistry, throw out the fancy isolators. Have a single large (many ampere) ignition switch controlled 'relay/solenoid' that connects the batteries together only when the engine is running. Add to that a low current switch in series with the ignition switch wire which will enable you to open that automatic engine-running connection between the two (sets of) batteries. (If you want to get fancy you can also add a switch that will allow you to connect the two sets of batteries when the engine is not running - - this to help cranking should your vehicle battery become "dead").
Maybe it would be more helpful to think of the electrical realities as "current follows voltage". Thus if both batteries have the same voltage and you connect them together, no current will flow. In the morning, after watching TV all night and having three pots of coffee and your eggs and bacon, when you fire up the Promaster (with its automatic ignition controlled relay/solenoid), the vehicle battery will have lots more voltage than your house battery. Two reasons - - the obvious one, you've been draining the house battery all night . . . the second, you will have just turned on a new source of voltage, the alternator. While your vehicle battery may have been charged and 'resting' (engine off) overnight at 13.6 volts, the moment you hit that starter, you've just enabled a pseudo battery, called your alternator, at 14.6 volts. Current follows voltage. Current immediately starts flowing to both your vehicle battery (trying to increase its terminal voltage from its resting value of 13.6 to the new alternator value of 14.6 volts and, importantly, it sees that discharged house battery, maybe at 12.9 volts, and does the same thing, tries to force that voltage to 14.6. And since the house battery is discharged (with a lower terminal voltage of 12.9 volts), you might (correctly) assume that the house battery is going to demand more current, maybe much more current, maybe too much current - - current that could damage your house battery (exceeding its maximum charge rate) or your alternator - - demanding more current from the alternator than it was designed to provide. How to stop this from happening (other than placing a fuse in series)?
The good news is that you may not have to do anything. Depending on the size of your house battery and how far it is placed from the alternator (and vehicle battery), the intrinsic resistance of the interconnecting wire may limit the maximum current that can be supplied to the house battery. We used this 'feature' in our now-retired CaRV in which our 245AH trunk mounted house battery was separated from the alternator by two 15' runs of
#10 wire. Even at its most discharged state, we never saw more than a 30 amp charge rate. There's a certain contributor on this forum (RD, are you ears ringing?) who reminds us "Keep It Simple". We don't listen to him because we're gadget freaks (and are trying to draft/add to our build thread details of our Not Very Simple electrical system). Just include an automatically ignition controlled relay/solenoid and you'll be happy.
One other thing to keep in mind when contemplating current flows . . . all of the above assumes that we were dealing with just three things: vehicle battery, house battery, and an alternator. But in our real world of travel and camping, there's the headlights, the previously mentioned coffee pot, TV, interior lighting etc . . . these additional loads will skew (add or offset) any battery charging/equalizing currents. We do not suggest worrying about this but note it only to highlight that the actual current flowing at any given instant may be a more complex sum of individual current demands. Generally we consider our charging source as 'charging our (house) batteries' but, if you're sitting at your campsite with those solar panels charging your house batteries and the refrigerator fires-up . . . what happens? Are the solar panels powering the frig? Or is it the house battery that powers the frig? A combination of both? Are the house batteries still being charged? And, the envelope please, the answer to these rhetorical questions is . . . yes, maybe, no and who knows? It can be any of these things and will vary depending on status of battery charge, intensity of sun/solar controller settings, and the magnitude of the load. Hope something in here is insightful.