Using a NEMA 5/15 plughead to charge from a battery charger that is outputting twice the amperage is subjecting yourself to a nontrivial risk of fatal shock. There is a reaon why NEMA requires a different (safer) plughead design when increasing from 120v 15A to 240v 30A. Your manufacturer says it's rated for 240v; but NEMA requires a different plughead for 240v (common called a dryer plug because most home dryers run on 240v 30A). They even helpfully explain in detail why there should be a different plughead on your product (so you don't plug the wrong thing into it!) while also stating that misuse of their noncomplaint plug can damage equipment or start a fire! We have found our 21 century lawndart!
I came here while researching where I could find the simpliest/most robust way to step down my 240v 50A home EV charger (hardwired to the house on a 60A circuit) to power 240v 30A woodworking tools so I was really interested in your setup. But using a 5/15 plughead on my charger and on my tools inside my garage? Hard, Hard, Hard pass.
Your battery's input is 2.15A based on the spec on your charger (input power, not your 6A output) and then you hooked up to an output charger that is 30-50A. My EV's max input is 114A. That means I'm getting 100% of the available charge on a standard level two 150A output charger and still getting 114A when I use a 200A, 250A, or faster output chargers. In the world of EVs; 2 or 4 wheels; it's impolite to hog a charger you don't need and your vehicle can't use because it slows down everyone else's trips while also not speeding up yours. Folks will drive on to another charger if they see it is in use (especially the slower ones) so you can't naturally presume that because they didn't stop, they didn't need it. (I suppose you can be the person that brings an overful shopping cart through the fast checkout lane at a not-too-busy time because the moment you walked by, there was no other customers waiting, but do you really want to be that guy?). A 120v 15A household outlet would still give you 100% available input power. Even if you charged from a 5A USB cable, you'd still be charging at 100% input because physics. I have a hard time believing that you can't find a plug near a park picnic area/little league field/concession stand/coffee shop etc.
I get the frustration. Every roadtrip I take, I find broken chargers; chargers that need strange 3rd party apps that take an hour to register ussing all my personal data and want to know everything on my phone; and chargers far away from any amenities or services. I carry a 120v 15A charger so I can charge at +3.5 miles to range/hr to keep me from getting permanently stuck somewhere. We live in a shared space of trying to make the world better through electrification. But my advice is: don't use your non-NEMA complaint plug in the rain... or at all.