I wonder if you have a WFCO converter? I also wonder if the interference is less outside your metal building?
There are those that say that a WFCO converter is the worst and that there are better brands out there, and cleaner than these, EMI wise. Progressive Electronics and IOTA claims to be. Although even WFCO claim to be class B compliant... by the way the standard isn't enforced (is there an FCC???)
Wiring acts like an antenna for the RF generated by the "sloppy" converter; that is your problem.
A possible way to reduce this is with a Ferrite bead (or two) placed around the 120vac input to the converter. You will need a "mix 31" bead; which is for the lower frequencies of the AM band. I know a split bead isn't as good, but you might want to try it and see. Either running a loop through the bead (two passes of 10ga will be an issue) or a second bead could work better. What the bead does is to add resistance (reluctance) so 60HZ passes easily, but high frequencies don't. See link.
A single loop in the power wire through the bead, might be something to experiment with, or multiple beads.
A bead placed on low voltage side (power to the radio) will be useless (or so I think)
A permanent solution will probably be a cleaner converter such as the Progressive or IOTA. Or ditch the converter and get a good (like a Magnum) inverter with built in charger.
Palomar Engineers Ferrite Split Bead FSB 1 1" ID Snap on RFI EFI Mix 31 | eBay
Your converter will burn up if you wrap it in foil, but a metal cage is a possibility, if however expensive and possibly not effective in solving the problem.