mbopp, that's the more usual way to attach, but this TT is a Outdoor RV(Nash) and they welded a couple of plates to the frame. They, BTW, were quite helpful with the axels, even discussing it with their engineers.
For the battery box, I'll use a factory latch lock, like what was on the semi box I'm chopping down. I don't have a pan brake.
I wasn't going to build a battery box until I lengthened the tongue for maneuverability. The TT is only 20'. We camp in forestry rec sites, and even with a 1 ton Quad cab long box, we should be able to get into more sites.