On my last boat, I had leak issues around the windows in the rear stateroom. The problem was the windows were kind of long'ish, and were right under the rear deck where we sat and came aboard, so the constant flexing of the deck always cracked the sealants.
I tried various silicones and sealants, and even something called "Cap't Tolley's Creeping Crack Cure". I figured with a catchy name like that, it had to work... but it didn't.
I ended up fixing the problem by diverting the water around the window with a rain gutter. I used these:
Essential Products RV & Marine
The gutters fixed the problem. However, they might not be the best aesthetic answer.
I got to thinking though, the gutters simply attached to the fiberglass with double-sided tape. It is not as strong as VHB (which means you could remove it later). So my idea is why not attach the foam gaskets to the RV with double-sided tape?
That might create a sufficiently waterproof seal from the gasket to the rear wall, leaving the front side of the gasket to (hopefully) seal the light bezel without application of any silicone.
And with the extra foam of the double-sided tape, it might allow the bezel to dig itself a "channel" into the gasket/double sided tape which might be sufficient to waterproof it.
If I were to try that kind of repair, I think I'd buy new gaskets. I did provide a source on the other thread for new gaskets from the manufacturer - I think they were only a couple bucks each.
For the LED light situation, I have not seen any replacement LED bulbs that are DoT approved for tail lights. Not saying they do not exist, but I have not seen any. I think if you were to replace the tail lights, you'd want them to be DoT approved for liability.
You need to make sure the LEDs are as bright as the original bulbs, but perhaps more importantly, the angle of view has to be sufficient.
Replacement LEDs for the side markers are probably not an issue, as they are not as important as the tail lights.
The Bargman light fixtures I bought were incandescent, but the manufacturer makes LED versions as well. In addition, they make replacement LED modules, so you can upgrade the incandescent fixture.
The only other issue is you may have to buy a LED compatible electronic turn signal flasher, as incredibly, the RV industry is still using 70yr old technology with the electro-mechanical flashers in some coaches.