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I really like your video's Jeff and a lot of thought put into your bypass relay. (thumbs up) I do have a question about the merge solenoid, so if your engine batteries are up to snuff (charged) and your coach batteries are also then there should be very small amount of current through the merge solenoid when the engine starts. So if the solenoid fried then that would mean either the engine batteries at one time were dead or the house batteries were dead when the engine was started and the merge switch was on and exceeded the current rating of the solenoid, your circuit will certainly take that solenoid out of the equation but if your engine batteries are low would that not defeat the purpose of the solenoid? I have a personal peeve with this the solenoid it should not be used unless you have a low engine battery to many times people use this all the time which causes other problems like if you have dead coach batteries, start the engine then turn on the switch the alternator has to charge both engine and coach batteries with a massive load and it could damage the alternator or leave it on all the time and damage engine batteries while boondocking. I think of that switch as a pair of automatic jumper cables that you don't have to hook and unhook but should only be used as such. I have dysgraphia so I'm sorry if I didn't make this better to understand.

Do you have that echo charger in the inverter compartment that trickle charges the engine batteries?

Thanks for your thoughts Jack and I guess what you are saying could be true. I think I am also fighting a parasitic load on my start batteries, I say this because if I do not leave the merge solenoid on the start batteries will drain pretty quickly. So really this is a band aid for now. I do not believe I have the echo charger but I have not really deeply investigated this issue. I need to dive deeper into why the start batteries are discharging. 
I may do an experiment based on what you are saying and measure to see how much current if any is drawn across the merge solenoid during starting, that is when both sets of batteries are fully charged. My only thought there is that you have much greater capacity in the house bank as opposed to the start bank, 6 batteries vs 2 batteries. It will be interesting to see what actually happens if you leave the merge solenoid on when starting and to see how much current is drawn across the merge solenoid. I have a nice DC clamp on meter that goes up to 400a DC so I can take some measurements. I think it’s also time for new battery cables, mine are not looking in the best shape and I suspect they are corroded internally.