If it is a thinlight florescent most any RV dealer will have the "Ballast" (really an inverter) and I've found replacement florescent tubes at Wal-mart, home, Depot, lowes, and any "Lighting" store you care to choose.
IDEA... Friend of mine found some LED backup lights.. He replaced teh Florescents with one of those. Till he mentioned it (Only one fixture was changed when I visited) I could not tell the difference, equally as bright.. I'm going to do the same mod.. About the same price as a pair of tubes.
Home is where I park it!