Question about Fusing

Without the battery as a load for the Controller, the controller is likely to blow.

However, properly wired and fused, the battery-controller fuse would only blow if the controller fries and presents a dead short to the battery.
Solution - never place loads on the controller, and then the only thing that would cause the battery-controller fuse to blow, is a bad controller.
If it's a 20A controller, wire it with 40A wire and fuse at 25A.

Thank you!

I had my panels briefly connected to the SCC prior to installing the batteries. This caused 20 volts to go through my 12 volt system. Did not ruin anything in the RV. It was the initial power on of my system and The SCC had not been set up yet, so perhaps that's why it pushed out so much.

On a different forum, there's a great debate about if this is safe. I'm of the camp that if it says it in the manual don't do it!!! The other camp has experimented by setting up SCC both PWM and MPPT and of the dozens that have claimed to do it, only one guy said he fried a charge controller that way. I'm more worried about spikes going into my 12 volt system and ruining something.

I can't comment on any RVs or your particular use, but automotive electronics is engineered to withstand double-battery operation for jump-starting. In other words, devices in cars must survive undamaged when powered for many seconds with +24VDC. This has been an automotive electronics requirement for >30 years.

There is another automotive electronics requirement called "load dump" that means that car electronics must survive brief excursions up to +35V or higher.

But I also agree with the Chrisski's comment: If the manual says don't do it, don't do it.