[chevyrestoguy]

I agree with BigSix, use seam sealer. Besides a longer lifespan, one of the other things I like about seam sealer versus any silicone product is the work-life and well it lays down. After you inject it into the seam, use your finger shape it and pack it in the gap, then use a wet rag to do a final pass and wipe up the excess. I have found that with silicone-based products, you get about two minutes of shaping and cleaning before it starts to "skin", and after that, you're making a mess and ruining the look.

If you're concerned about the main interface between the bedside and the wheel tub (in between the bolts on the tub flange), then get some rope caulk from Lowes or Home Depot and attach it to the tub before you bolt it on. Do a full run, even over the bolt holes. Once you get the tub in place, trim the caulk in the bolt holes with an Exacto. Bolt the tubs in, and then seam seal the exposed gap from the top.