I replaced all of the inside lights in my TT with the 15 LED 5050 warm white panels that you can get on eBay at 10 for 37.50. Slightly less light output than the 921 bulbs, but great. The lights come with different connectors to replace different bulbs. They attach with double sided 3M tape. A bit fiddly to install.
I mostly camp where there are no hookups,so this was definitely worthwhile for me. If I camped with full hookup only, I don't know if I would have bothered. It is nice to leave more than one light on at night.