OK, following up.

The circuit is now 20A, and terminates in a drawn metal junction box, with ground lug, and a cover, with the KO still covered. I've got a length of liquidtight conduit and two suitable end-connectors, along with some 12 AWG THHN, that will be waiting in my parts drawer when all the rest of this sleigh ride is finished.

I've taken to heart that ImOld's point that MH water tanks typically are 30gal; therefore even if this one originally came with a lowboy, it would have been built to support it. I will therefore be proceeding using the tank that I have. Having no way to really calculate my load bearing capacity, all I can do is spread the weight out as far as I can, cross my fingers, and stand back the first time I fill the tank and tub.

The plan is to open more subfloor cavity, into the MBR closet and burrowing under the kitchen counter, to expose 4 of those transverse 2x4's, and then lay down two new 12' long joists on top of them. This in addition to the sistered-in rim joist and nailer that probably can't go in as continuous pieces, although I'll try.