I just upgraded the six 921 bulbs in my 625D pop-up to these:
LED 6 CHIP 921 REPLACEMENT RV BULB | eBay
I puchased these because they have the circuitry to drive the LED bulbs properly with a constant current. These gives longer LED life, better reliabilty and constant brightness as the battery voltage changes.
Installation couldn't have been simplier, slide the lens off, pull the 921 bulb, push in the LED replacement, slide the lens back on.
To my eye the LED replacements are not quite as bright as the bulbs. But the bulbs look really orange-yellow compared to the white of the LEDs.
The main reason I changed to LED was that if we had two of the lights on and turned on the third the other two would dim because of the current draw. Now that doesn't happen.
Overall I am very happy with these LED replacements.