The triple tail lights
of the 5500 are called "Grote 51342", or "Monarch 4.25" triple lights. Common on many Airstream trailers for some time, with slightly different housings and color (the lens and internals are the same).
You might want to Google "Grote 51342 LED conversion" or similar key words.
Here are a couple links I quickly found:
Sealed LED light kit for old 4.25 Monarch LED-kit2 [LED-kit2] - $179.95 : Out-of-Doors Mart!, More Airstream Parts on-line than anyone!
LED Tail Light Kit for 4.25 Monarch Airstream
For some reason my 5500 has double Monarch lights instead of the triple I've seen on every other 5500. Not sure why.
In any case, here's how I converted mine to LED: I bought some red SMD leds (larger ones for the stop/turn, smaller ones for running light) and wired them to 1157 bulb bases. Stuck the SMD inside the housing with 3M double face tape. Works perfectly, they are pretty bright, and I can go back to regular bulbs if I need to. For the backup lights I simply replaced the regular 1156 bulbs with some white 1156 LED.
Also, I invite you to list your 5500 on the Trillium
5500 Registry. All you need is you serial number.