Put the cap ANYWHERE on the 12 volt line between the charging unit and the load. Certainly you can access something, somewhere. The line just has to have minimum of 14 gauge or larger original rig wiring. Maybe at the major fuse panel? Heck if nothing else, connect the cap to the REFRIGERATOR + and - 12 volt wires and see if that works. Complicated explanation, but capacitors used for ripple suppression can only get "So Far" from the origin and display of the ripple before they lose effectiveness. But try the refrigerator connection.
It's a cheap, effective fix if the connection is not made "outside of range". If the reefer connection does not work out, try connecting the cap directly to the battery or battery switch. You'll be amazed at how easy all this turns out to be.
Secondly, inside the bathroom where the flickering monster connects to power, can a tiny axial capacitor be wired to the positive chassis wire lead then the other cap wire to ground? The capacitor size I'm thinking of is the diameter of a dime and as long as your thumb nail? This would be a .47uf size cap and it would address an entirely different ripple frequency. Note that BOTH caps act to stabilize and "clean up" the entire coach 12 volt system, when the batteries are seeing converter power AND alternator recharge power. I used to DOUBLE the life of those horrid Thin-Lite fluorescent tubes doing this modification.