FWIW, the RVI toad charger is nothing special, imo. You are basically paying for the ability to monitor your toad battery from the proprietary RVI hub and tablet.
It seems to actually have more of a voltage drop than a simple fuse/diode would.
I use the RVI because it came with the brake system I bought used. I certainly wouldn't pay $120 (I don't actually know how much it is, since they don't show a price until you log, which is a bit sus).
I agree with Willald, put a voltmeter on the TOAD battery with and without the motorhome running. What's it read? Any chance you just need to replace a fuse in the charge line (should be very close to the TOAD battery itself)?