Looked one over identical to yours years back, if the owner would have changed the oil regularly I would have purchased it. It had 70k miles and had a rusty drain plug bolt and a nasty oil filter, then there was the stress crack in the windshield frame next to the gasket. Wish it was a better example, loved that floor plan.
Your 643 will do fine with Dextron, not always the best plan to change a vintage transmission over to syn. You can change the Dextron yearly for a long time before you approach the cost of a syn oil change over. I did go with Amsoil Tork Shift for the MD 3060 upgrade I did to my Wanderlodge. I own a transmission reman facility so I do have some insight into the requirements of Allisons. Motor homes aren't really much of a challenge for 643 as long as it doesn't get overheated.
