One would go from DC to 400 Hz back to DC. I would agree, it would not make a heck of a
lot of sense to go to 400 Hz, and then try to get it back to 60 Hz.
In fact, it would make no sense at all. I was just wondering if there
would be any significant advantage to convert DC to 400 Hz and a higher
voltage for a "long distance" transmission line from PV panels off to a
relatively far off battery bank.

Those transformers were from junked
Jet planes, and we were told that it was more efficient and took
lighter weight wiring and everything else to go to 400 Hz electricity
at higher voltages, and then convert down if lower voltage DC was
needed. Back then, solid state electronics were not all that common in
jets. It was just out of the Boat Anchor Tube Rigs days.

Seems like we are always looking for the cheapest, most reliable,
longest lasting solutions, and I was bringing up 400 Hz just for
thinking purposes. :-) We are always stuck with Ohm's Law, and any way
of moving electrical energy has some trade off. Higher voltages,
smaller wires. But up and then back down always takes some of our
energy. Same, I would imagine, with going from DC to AC, and then back
to DC. No free lunches. But the cost of good copper wire is getting
astronomical. :-(