Landy, you (and this subforum in general) are a wonder!
Your charging data are very useful. I can set up my batteries as 2 or 3 banks, to charge them at a higher rate.
I didn't know, either, about putting on a load while charging, as a test. Whether my HF inverter (nominal 1500watts, very finicky) will handle a microwave, I dunno. But I have two here, a half pint and a normal one, so I can give it a shot.
I also have a length of heating coil, which used to be calibrated to 12v 25 amps, but is much shorter now. Does that mean more amps, or fewer?
I've been googling and checking my (large) stock of extension cords. Looks like 16ga is ok for the PD if under 50 feet, so I might make one with ends to connect the 20a to an RV 30a outlet for RV park usage.
When driveway docking is a little more tricky. I can get a 20a to 15a adapter, and I have a long 14ga cord, but the question will be whether I will blow a circuit if I plug into a house. It's pretty unusual that I would show up with dead batteries if I have alternator charging. If I do, I guess I can ask people where they plug in their microwave / space heater / AC...