A Separett toilet has swappable containers to give more volume. Very good lids.
Will the shower drain and plumbing go straight down through the floor? Or will the tray be elevated to allow pipes to go underneath? How will you fix the toilet down to the wet shower tray (if that's what you meant?) If you have plumbing under the tray will you need to lift the tray up again?
Our Mog camper had an Eber D5WS correctly plumbed in to the engine circuit, but then a pair of Ts to a loop running all around the camper floor to calorifier and air blown heat exchanger. Every time the taps on those Ts were opened to heat both engine and camper we got air in the camper circuit which meant it didn't work properly until it was bled which was a nightmare. We bought this self buid camper already built and used. I don't know why the air did that, but we never tried to heat the engine with it again although we maybe only ever had -15C or something. Or use easy start apart from blowing the tyres onto the rims.
A UK Eberspacher engineer (employed by them) told me the length of run round a camper meant the engine circuit wouldn't correctly pressurise so you should use a heat exchanger between circuits. His main work was high end boats so definitely didn't want to damage anything.
Although it may have been possible to have one heater for engine and camper we now just have one for the camper.
Spares though, that's another thing. To bring servicing up to date on that Mog camper we had to clean the D5WS quite a few times, then renew parts, and later for instance we had to swap the air fan. Parts cost a fortune. Opening up the burner compartment was secured with screws that were really soft and easily rounded. Not a part I would have brought as a spare. If you look into a few of the bigger spare parts costs you might think it not a stupid idea to carry a second whole unit just for spares?
Using gas for heating might use more than is sensible on an overland truck especially when you need different bottles in different countries. We did look at a FussMobil truck that a family had used around Canada with gas heating and they needed new bottles every two weeks. Maybe ok within one country, but how many standards are there just within the EU? Even one brand you can get in both France and Spain has different fittings in each.