I was looking at the newer Winnebago small (tiny?) Realta motorhome - the successor for the LeSharo. The toilet in it appeared to have the offset lid like the original Argosy toilet. I couldn't do any measurements since it was more or less permananetly installed and in a dealer's lot, but it might be worth investigating more.
A friend also mentioned that I might check out the local marine stores for both toilets and holding tank manufacturing experts. It seems that the holding tank in every boat made is different than all others. Just like snowflakes. The boats have at least as much of a problem with their holding tanks as land-based vehicles.