Help why is my battery terminal corroded and sparking?

I will ask the obvious

Is the connection tight?

^What Lynmor said.
In particular #3. Plugged in continuously charging. Does your converter also function as a battery maintainer?
If it does not, it is overcharging the batteries due to little to no draw on them and full charging amperage.
1 of 2 Solutions there.
1. Disconnect batteries when not in use and periodically charge them or put a maintainer in them.
2. Depending on your converter, you can add a Charge Wizard to progressive dynamics converters. Possibly similar for other brands that monitor voltage and maintain the batteries.

Corrosion is leakage. May not come back if not overcharging. Or may.
In the past I’ve cleaned up old batteries around the terminal base and put a coat of rtv sealant around them. Stops minor leakage for a while.

Lynnmor wrote:
1. Battery is beginning to fail causing electrolyte to cook out.

I would not condemn the battery just yet, but I will bet it is a couple of years old !

rvshrinker wrote:
Last looked at these batteries one month ago. Today before our trip there is a small amount of blue green corroding at the terminal, and along 3 red wires connecting to it at their terminals.

Corrosion forming around lead acid battery terminals is from a small amount of gas that is leaking around the terminals. It is impossible to prevent this leakage as a battery ages. You can buy those pricey battery post "donuts" or just gob a large amount of Vaseline on the post and terminal/clamp. Remove the terminal so that the Vaseline get under the terminal/clamp right up close the the seam between the post and the case.

BEFORE YOU DO THIS, clean the terminals, cable ends and the ENTIRE BATTERY OUTSIDE CASE with baking soda and water. Rinse well with clear water.

rvshrinker wrote:
Also there is a small amount of sparking when the red wire is brought to the terminal. Never seen this before. What's going on?

If it is small, ignore it. It just means that something is turned on when you connect it.

Why would you turn off your solar system if you have a quality controller? Solar controllers are designed to properly maintain batteries!

Did you leave a light on? Fan? Radio? Fridge?

1. Battery is beginning to fail causing electrolyte to cook out.
2. There is a bad connection between metal parts.
3. Battery is being over charged.
4. There is a load turned on while connecting wire.
5. The converter is on causing the spark.

Choose one or more of the above.

The vehicle is unplugged so there's no other source of power. Solar was turned off. Batteries have adequate water. Vehicle had been kept plugged in on 30 amps.

I unhooked the negative terminals and sparking stopped.

I dont know:h

Are you using a battery terminal sealer?