As I understand it both low and high beams use H7 socket, so you need an adaptor to make room for the heat sinks/fans of LED lights. The no-brainer choice would be to use the OpenGlass kit which seems to come with everything ( Promaster, Hymer Aktiv, and Winnebago Solis Accessories
However the best LED for the Promaster looks like the Morimoto 2Stroke 3.0 with its H7 adaptor--only that's a general purpose adaptor. I don't know whether it works in the Promaster headlight nacelle. Especially since the OpenGlass kit provides different adapters for the low and high beam sockets, even though both are H7s.
You can get LED bulbs that fit without adaptors because they have no bulgy heat sinks. They also provide less illumination and burn out in a year, or so I've heard. That does not sound like a solution to me, especially since you have to take out the Promaster headlight nacelle to change the bulbs.
For other LED bulbs I get the impression that they require modifying things in there--gluing/drilling--which I don't want to do.
My biggest concern is making sure they don't blind oncoming drivers. The Morimoto is carefully designed to exactly mimic the optics of a halogen bulb in terms of beam pattern. I hope the OpenGlass does the same but I don't know. I've queried them. The Morimoto also has a unique cooling system that blows air onto the LEDs taken from within the nacelle. That would melt ice on the nacelle's exterior, though that's not much a consideration for me given where I live and go camping. But Yellowstone might make that relevant.
I'm climbing up this learning curve bit by bit. If anyone has LED headlight experience to share I'm all ears. We have the factory fog lights BTW. Morimoto makes H11 bulbs that I guess would fit but they also make a "universal fit" complete fog light in housing. I'd love to know if that could go in there easily. They make fog lights with a lot of different custom brackets, but not one for the Promaster AFAIK. Would the Universal model fit?