RV Repair: 1989 Winnebago transmission, torque converter, 23f

Question
Hi,
I own a 1989 23f Winnebago on a Chevy chassis.The motor is a 454 and the transmission is a 3 speed TH400 (13-bolts), the mileage is 48000k. My problem is that when I take off from a standstill, it will normaly shift smoothly through all the 3 gears and top out @ 55mph, with the engine reving very high. No slipping of the transmission itself. Its seems like the Overdrive is not kicking in, but if I bring it to a tranny shop, they will charge me an arm and a leg, to just look at it. Is there a way to troubleshoot the overdrive particularly, to maybe narrow down the problem?

Answer
In my opinion, I think the torque converter is shot.  You have a 3sp.  It shifts fine through all three gears.  But you dont feel OD.  OD would mean a 4th gear on a 3 sp.  Since you don't have a 4 speed, OD must be a lock up torque converter.  IF that TC is not locking, you wont feel like OD is working and the top speed would be greatly reduced.  

Good luck