Thanks to everyone for their input. I think what was happening is that sometimes copper tubing is defined by some by OD, and sometimes it's defined by some by ID - regardless of the correct way of doing it. So finally, I got a 3/8" Forged nut to see if it would fit on the threads of the gas orifice in this new Atwood water heater. It did. To me, that now means I need to run 3/8" OD (1/4" ID) to insure I have the right size to fit the orifice in the water heater - and - incidentally - that will also marry up with that shut-off valve below the trailer and match the line coming in from the main split!
Thank you to all for your input and advice. This was driving me nuts until I got all of the replies and was able to work thru it. You guys are GREAT!!!
My Sunday project is ON!!!