Is there some kind of an anode that I have to have, if I use a non-marine aluminum manifold? Think I read that somewhere.
Thanks!

no .....just flush after every salt water use ....

I disagree. I had 420's that came with aluminum manifolds and I flushed them for 15 minutes avfter every use. In 3-4 years a pit formed underneath the thermostat and bore a hole to the point they leaked. I replaced them with manifolds that had a brass sleeve in the crossover.

Sounds like you had a ground issue turning your aluminum manifold into a battery. Aluminum manifolds on trailered boats are fine if properly flushed.

Also makes another point for shutting off your batteries after use.