Some pictures would be useful but I can give you some pointers.
You want to carefully lift up the, presumably, lino that is in the bathroom.
You also need to head outside and under the 'van to carefully remove any insulation. This will normally be covered in black plastic wrap.
Then it should be a case of cutting out the rotted area and replacing with new chip board. Make sure the new stuff is mairne grade.
Then put back insulation and lino.
Other things to note, keep an eye out for any pipework that may run under the bathroom when cutting out the rotten wood. Things can easily be made worse
Send us some pics and let us know how you are getting on
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