GeorgetownJim,
I, too, fail to see how the toilet inlet pipe extending too far into the black tank would create an odor problem anywhere. I could see it reducing the time between black tank dumps.
If you're getting an odor from "there", which I'm assuming is a point difficult to locate, I would guess that your vent pipe to the roof is either cracked or loose, either at the tank connection or somewhere inside the wall. From the vent outlet on the roof, try to figure out which wall has the vent pipe in it and see if you can locate the odor to that wall. Perhaps noticing that it is stronger inside a cabinet or drawer against the wall, or where that wall meets an abutting wall.
Good Luck and keep us posted on your results,
~Rick