dissipating the heat is a huge as of now unresolvable issue with lamps stronger than 2.5 watts housed in an RV fixture.
This I can believe. The small size of the fixture is really working not to your favor. I think you may have to cobble up your own fixtures to overcome the trapped heat issue.
FWIW, I mounted my lesser 36 or 48 SMD panels on a piece of sheet aluminum. Even though my system voltage reaches 14.5 and above, all of the cheap panel LED's keep functioning. I assume that the aluminum sheet helps with dissipation.
How much current and which LED configuration are you using? These are the whole casa lights you were working on?