From reading everything available on the web pages for the product, (the installation instructions link is broken) nothing in that box does anything different than a fuse, diode and an LED would do. It limits current to 15 amps by a fuse, no second voltage regulator. The F.A.Q. page is carefully worded and explains exactly what I said earlier, two healthy batteries will equalize. As one is used, the other will have current flow to keep them equalized. Adding a charging device, alternator, battery charger, etc. will maintain both batteries to the same level.
Calling the device a "Towed Battery Charger" is incorrect, the RV engine alternator is providing the charge, the voltage regulator is regulating the charging voltage, the nature of battery science is regulating the charge to the towed. The device only adds a diode and an LED, a lot of money for those components. Install an auto reset circuit breaker to prevent heavy current flow back from the towed battery to the RV while cranking the starter, if you put a diode in the circuit even that would be stopped.
I still think the feature of the "premium metal casing" is ridiculous, if you were to locate it near a high heat source like the exhaust manifold, the wire insulation would melt before the "premium metal casing" would anyway.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI