Check to see if your VSR is directional, and hooked up in the right direction.
Solenoids make for cheap and reliable isolators. Usually under $20 from a battery shop. You want a “normally open” one as opposed to a
“normally closed” one. It will take a trigger wire to the fuse box, to something that is fused, but only works when the key is turned on.
When that wire is energized (engine running), it will connect both batteries so the house battery will be charged. When the key is off, they
will be disconnected from each other.
No fancy electronics to go bad, solenoids are time tested as pretty darned reliable. The one I have in my current van has been working
reliably for over 10 years now.
I would also get a load test done on your house battery. If your VSR isn’t working properly, it could have damaged your house battery.
Also worth mentioning in case you need to replace your house battery, I have been buying almost new deep cycle batteries from auto wrecking yards for a while now, usually for under $20. My last one lasted for a little over 7 years, not bad for $18. Just make sure you’re getting a deep cycle battery and not a starter battery.
"Proper planning, preparation, and equipment, ensure proper performance." ~ Road Warrior