I believe the SAE connector that Zamp uses is rated at 15 amps. I used it with a 160 watt panel that hits 8.5 amps on a good day, and it held up OK. I had more of a problem with it loosening up after many plug & unplugging. I crimped it with a pair of pliers, and it was still going when I sold the trailer.
I'd love to find a 25 - 30 amp, high quality waterproof 12V connector that includes a panel mount version, but as yet, haven't found one.
Following your lead, I had a Zamp socket installed on my 17B for external solar input.
What do you suggest for a plug and cable adequate to handle the current load?
I've yet to find a plug rated for much current.
Do you have your 21' yet?