I was thinking of pruning a couple of super bright LED reverse lights on my boat trailer - but would be a 3.5A draw

I have no idea how the towing electrics are wired/fused

I could run it off the fridge circuit with a relay if needed.

Does anyone know what the reverse circuit is rated at ?

If not, I guess I'll just try it our & see what blows

IMO you would be better using the reverse circuit to switch a relay and use the relay contacts to feed your reverse lights.
You could then take the feed for the additional lights from one of the permanent +12v feeds to the sockets. Doing this means you won't upset either of the lighting/tow ECU's.
Tis what I would do.

Thanks - confirmed my inclination - relay & use the fridge circuit