There are many repair kits for sale on 'tinternet'. Varied prices etc.
I would ask another question as this is a recurring problem on Motor Home Forums. It may be best to ask, has anyone had success with DIY jobs? and the name of the particular product the used to do it with. That way you will get good and bad answers.
Good Luck, and have a word with your travelling campanion, and put him/her on a diet. LOL
This comment will not help the OP but as a person who has built his own campervan, I know only too well that one of the most important aspects of the entire build is to make a fully supporting frame that the shower tray fits into so that is totally supported.
The problem with a lot of professionally built motorhomes and caravans that I have seen is that so many of them don't pay even lip service to ensure their shower trays are fully supported!!
I don't how old your motorhome is, but if it isn't that old I would take the matter up with the manufacturer or dealer you bought it from.
------------- There is a great World to be found out there, but by the very day, to find where it is becomes more of a challenge!
There is a previous thread on this subject, I seem to remember a specialist company that fixed shower trays, Speedcoat comes to mind, I if it’s not the correct name come back to me and I will see if I still have the details