I believe your old trickle-charger should work.
I needed a replacement trickle-charger for my Minnie Winnie 22M, so I just bought this:
I am very happy with its performance. When plugged into Shore Power, I use the cigarette-lighter adapter to hook into the engine starting battery, and plug the 120 VAC side into one of the interior outlets. On the Mercedes, I don't know if your cigarette lighters stay connected to your engine starting battery when the key is off, but if not, the charger above also comes with other adapters.
Welcome to the Forum.