how do I replace to rear fog lights?

Might be cheaper if that is the case to have new fog lights fitted. I'm sure they could pick up power from the existing lights so the same dash switch operates them. I certainly would be cheaper than dismantling the furniture ect in the back.
I can't see that a coach builder would not leave access to lights unless this unit is 'Home built' from a normal van, then whoever built certainly needs to be avoided for advice.
If factory built then contact a caravan service centre see if you can find any information who manufactured it they may even tell you how to get to them.
Good luck!

bob UK
March 2014