Not a Chausson, but on our Hymer, access is by a plate screwed to the locker ceiling, underneath the shower tray. If you have a double floor, then there is a good chance you may have similar access. If not, then like John advised, the shower tray will/may need to be removed.
I looked at taking the walls apart but each time i took one thing off there was another and another so i stopped.
I will get some mats out tomorrow and look underneath to see if there is any access that way ?
If not i might have to cut an access panel underneath. The new part is only 89 p
We managed to reach ours from the kitchen area but different van.
But what we realised was that the waste hose was not flexible enough and constantly pulling on the trap as the waste tank moved whilst travelling.................after replacing twice we now slacken off the trap with the central screw (so it can move) and only tighten up before taking a shower
On our old flash 03 there was an access panel in the back under the lower bunk. There were 2 plastic grills which could be removed to look under the shower tray you might have similar? Try looking at a low level in your garage if the shower is next to it