Diesel Don,
I may run into the same problem as you but I found (I think!) that the problem in my case was not the trip odometer itself but the pot metal wheel (which is the first gray colored wheel in the actual odometer when you have the cover off and can look at it). The shaft of the main odometer must be pressed into that gray pot metal wheel... which as it turns, clicks of the tenths of miles on the trip odometer. I found that when I initially repaired it, I did not press the brass fitting on the opposite end of the shaft tight enough against the sheet metal "box" that the whole contraption fits into... that caused the brass sprocket which is driven by the plastic worm gear to work it's way outboard which then but a bind on the worm gear and ultimately broke the small tip off of it where it "carries" on one the end of the worm gear. My solution: screw a small wood screw into that end through the housing and then hacksawed the head off. Now that fixed the worm gear. Next I used a C-clamp to press the brass retainer on the opposite end of the housing so that it would keep the brass sprocket snug against the worm gear. I then tested it with a drill and it worked flawlessly. I just today did my first refill after a light week of driving and with 248.9 showing on the trip odometer, she held about 10.8 gallons. The mileage worked out to be 23.036 mpg so I was somewhat happy. I think that is the first time my odometer worked for the whole week. That is what I like about my ol' gal..... there's nothing much that can break or wear out that I cannot fix myself :}!