Allison transmission shifting problem

I just bought a 1976 Flxible Bus/RV conversion a week ago near Nashville. Everything seemed fine, brother said I was doing 70 on I-24 (speedometer seriously off). 10 or so miles north of Nashville on I-65 I noticed that it had probably down-shifted on a grade, but never shifted back up to 3rd. It's a 6V Detroit, Allison V-731 cable-shift transmission. For about 50 miles to Bowling Green, Kentucky, I probably didn't get over 50 MPH. I tried maybe 3-4 times manually shifting down to 2nd, and it would shift down, rev and accelerate some, then seemed to fall flat on it's face....like the engine died, which it didn't. I could shift back to Drive and it would keep going. Somewhere in all this (first time I've ever driven a vehicle like this), I noticed the tach hand jumping once or twice, but really didn't pay it much attention. At Bowling Green, it would no longer shift manually to 2nd, and the tach had quit working entirely....my manual doesn't even indicate the bus had a tach, and both the tach and the speedometer are "new", had been replaced. I had to get off I-65 to keep me and my brother following from getting ran over, and took God-awful back-roads 30 miles or more to Glasgow, then got on the Cumberland Parkway, which has very little traffic. I was able to drive to my home-town, Edmonton, with no indication of problems other than being SLOW. My brother said Bowling Green to Edmonton was 30-40 MPH.
When I got off the Parkway, I put the shift lever in 1st (bus will do the same in 1st, 2nd, or drive), put my foot to the floor and held it a while to see if the engine would rack up the RPMs. It didn't. Makes no difference where I put the shifter. I DON'T think I was in 1st gear all the way from Tennessee. Transmission showed low, so put in 3 quarts in Bowling Green. No difference.
One thing that may be important is that at some point before the previous owner, the bus was converted to pneumatic (air) throttle. There is a transmission modulator split off the pressure line from the pedal to the engine throttle actuator. I'm new to all this stuff, but wonder if that's the problem. Reverse works fine, and neutral, and correct with the selector on the gear shift. It is, by the way, a cable shift. If the pedal is pressed to the floor in neutral, the engine seems to build high RPMs.
Any suggestions?

Are you sure this is a transmission problem? If the engine isn't producing full power, the tranny wouldn't upshift because the engine is laboring at a lower hp/torque. Could be as simple as a dirty fuel filter, or as bad as failed injector(s). Lack of either fuel or air will reduce power and make it seem as though you can't get into high gear.  However, in neutral with no load on the engine, it still revs up fine.

I agree with Gary on this one.  First thing I would do is change ALL the fuel filters and try it again.  Especially is the bus sat a while the filters may have clogged up.  If the filters look like they are full of black jello you have algae growing in the fuel tank and should have it cleaned out. Those transmissions are pretty simple and reliable and it sounds like it is shifting.  They depend on the input shaft for a lot of functions so if the engine can't spin it fast enough it won't work properly.

Thanks Gary and boatbuilder. I'll check the fuel filters and open up the air filter to make sure it's clean. I know the transmission depends on engine RPMs, and at first thought the tach had something to do with it. But besides the bus maybe never having a tach, there isn't a feed from the source (generator ?) to the transmission. I could see the fuel filters being a reason it fell on it's face after the RPMs got up. I would have thought it would still down-shift to 2nd by moving the selector to 2nd.
Thanks again guys.

Newer Allisons know they are smarter than the driver and will refuse to shift to an inappropriate gear, whether too high or too low for the engine RPMs and power demand. My Allison 3000 shows a "target gear" (the selected one) and an actual gear (what the tranny is currently using). I don't know if the old V731 is that smart or not, but most automatics won't downshift to a gear that might cause the engine to overrev.

In any case, it's worth a try before getting into more serious tranny repair scenarios.

not sure on the V731 but the older MT and HT series would not let the transmission downshift if it would over rev the engine.  They would also upshift, even if you had selected a lower gear, to prevent over revving.

The 730-731 transmissions are "basic" automatics (4 speeds) but reliable.. I would first go to the fuel filters..Is the engine a Detroit 6V71 or 6V92 NA or turbo'ed??  Also 70 is way to fast for that combination to yield any decent mileage, your final drive ratio is too low to support those speeds,, if you had a reliable tach you would be seeing RPMs in the 22 to 23 hundred range, that's near governed speed for those engines and they don't live long in that neighborhood.>>>Dan  ( Go to Bus Conversion Magazine bulletin board for discussions by the guru's on your engine & transmission)

Thanks to all for your suggestions, and it did indeed turn out to be the fuel filters. I just got insurance and tags yesterday, put the tags on and took off (installed the new filters a few days ago), and it ran and shifted fine. Cheap fix, this time.
Thank you all again. Now I'm going to post a new topic on how to check the tach which quit working at the same time.