On a related subject, has anybody used a desktop with a 12V
power supply for radio modem/weatherfax use? Using it with
an inverter is out due to interference.

No but try different inverters, some interfere more than others. I
have several small inverters in the 150 to 300 watt range for powering
laptops. Most of the time reception is OK on all but the weakest
signals. Pactor 2 and 3 have an amazing ability to pull usable signals
out of the noise. Otherwise, charge up your laptop in advance and run
from the internal battery when you have a reception problem.