The Perfect Switch isolator has no V drop but is expensive.
The simple constant duty relay works but at night with the lights on and under a big electrical load, it will in my experience drain your TC house batteries way down.
The solution I plan to get is a smart solenoid that will sense when the start battery side voltage is dropping to low and will break the circuit.
A manual switch with a volt meter to tell you when to break the TC circuit at the solenoid would suffice.
I have had this unexpected situation happen a few times and it left me disappointed to find I was much lower on TC house charge than expected when pulling into a campground late at night.
I guess the house batteries fool the alternator voltage regulator into thinking it has more battery voltage than it actually has so it doesn't kick into a higher voltage output condition.
I'm sure there is a person or two around these parts that can explain this better than I.