I don't know where to buy 12v LED marker light assemblies. What comes to mind is replacing the incandescent bulbs inside your existing lenses with LED bulbs.
Concerning leaking lenses, on our first motor home, I gooped a massive amount of conventional outdoor silicone caulk behind each lens (focused around the hole for the wires) then immediately screwed down the lens, then wiped off the ooze. I never had a leak again through any of them.
Our rig today, the PC factory seals each lens around the outside. So far, that seems to work.
So there you have two different methods to consider on top of the others.
One thing seems to be unreliable, are the gaskets that come with some lenses. I would toss them.