Welcome to the forum.
I can't answer your question about the toilet compartment as our van doesn't have one. I hope someone replies with info and pics which help or you get the answer through the Facebook page where I think you have already posed a request. If these fail you could check out pictures of some of those that are/have been for sale on Ebay etc. Maybe even get to see one and check it out.
Alternatively, why not make a visit to the annual get-together in August if you can - you don't need to bring your van if it's not ready (or even stay over). You will probably get a good chance to closely inspect and measure several toilet compartments and get some ideas for equipment etc.
Hope that helps
An Esterel is for holidays, not just for sleepovers !