Not sure why to isolate Matt? Zinc is pretty close to aluminum on the galvanic scale- and usually corrodes first! That's why we use it on the hull - to protect aluminum.
That said, zinc chromate primer paint? thin sheets of plastic film? some bottom paint?
I'd say corrosion would be as low a rate as you can find? Even aluminum to aluminum will corrode faster than zinc to aluminum.
We haven't taken the old brackets off yet, that should happen tomorrow.
The trailer shop that sold him the parts said to put 3 layers of duct tape on the rails to stave off electrolysis.
I think that will hold salt water and accelerate corrosion. That is what we would like to slow or stop.
Would you also recommend duct tape, leave it alone or something else?
Different application. But ... The best of the snowmobile trailer manufacturers, whose products spend their road time in environments consisting of heavy, liquefied, and often sticky salt with lots of freeze/thaw cycles use a nonreactive, thin strip of UMHW or equivalent in between the axle and frame. It's generally about 1/8" thick, which is to say, it's not a "film" so much as a "pad".
If the separator fails, the results can get ugly (e.g. the rusty galvanized axle pad punches through corroded aluminum trailer frame ... makes for an interesting ride home with the wheels rubbing through the plywood trailer deck from below).
Triton Trailers has a pretty decent online parts fiche and i've always had good luck with their distributor network. It's pretty far from where you're at but Fisk Distributing in NY is staffed with knowledgeable and decent people if you want to talk with them about ordering the Triton parts, but I think you could also make your own cheaper and more custom, from stock sourced locally.