We have a factory installed HT in an owner version of L400.
The location was described in a post above so I would not repeat.
But to add: If you install a HT in the shower stall, be aware that the new hose that will discharge the toilet into the HT goes a long distance from the toilet, under the sink into the stall, and the up to be connected at the top of the tank. So, ensure to make at least two inspection ports
in the stall's wall at this location. This will allow inspection of this end if the hose, plus option of dismantling and reattaching the hose.
The standard inspection port as installed by Lagoon
is useless as it shows only the middle of the HT.
The very long path if the discharging hose makes sure that it will be stuck after 3-4 years of use and then the suggested ports
will be very useful.
You will also need a breething vent for the tank and if you want to be able to pump
it out at a service
station, you will need an access from the deck to the HT.