I am a fresh owner of a Bavaria 40 - 2001. The boat is fitted with only one holding tank (larger heads next to companionway).
What size, shape, material and where have other owners installed a holding tank for forward heads. We would not need sea dumping by gravitation, so the tank does not have to be located above the waterline. In my cruising area there are plenty of suction stations available.
BR,
Tommi - a fresh B40 skipper