What is best sealant when replacing clearance lights

Drew is right! If you have previously sealed the fixtures with silicone, you'll need to remove all traces of it before sealing again. This is one of the downsides of using silicone. Another is the degrading in sunlight and weather. The polymer sealants that are available are heads and shoulders above acrylic and silicone.

Since I discovered Geocel Tripolymer sealants, that is all I use on anything that is exposed to weather.

I been using quality clear silicone for jobs like that for years on my RV's. No problems.

I use the clear sealant from the hardware store. Be sure to observe your removed lens for a possible WEEP hole which should be on the botttom to be able to drain off any water that might get into your light assembly. Otherwise the area may fill up to where your light socket is and will then drain inside your RV... Some lens have the weep hole and some don't...

Roy Ken

Anytime I add or fix something attached to the sides, I use the black butyl tape behind the fixture, then use an acrylic? window caulk (typical for use with home windows-flexible and durable, easier to remove if necessary-specifically, DAP Dynaflex 230 clear). The acrylic is so much nicer to deal with than silicone. Also it goes on white but dries clear, so it's easier to see when working with it. x2 about leaving a way for any wayward moisture to escape out the bottom.

Preferably not actual silicone. It seals okay, but if it ever needs replacement or touching up it's a long, hard process as every bit of the old must be removed. Pretty much nothing sticks to cured silicone, not even more silicone.

Geocel Proflex RV is the "standard" sealant for this sort of thing. There are other polyurethane sealants that are very similar and perhaps more readily available for less money; I've used OSI Quad from a big box store with decent success. Somewhat less standard, but in my rather limited experience also effective, is to use plumber's putty under small attachments like clearance lights and cargo bay door holders.

3M 4400 would be another good, tried-and-true option. 3M 5200 would also work, but you'd pretty much never be able to remove the lights if ever needed.

Clear silicone top and sides leave a little open on bottom in case water gets in it can drain out