RV Repair: engine reving, fan clutch, seebreeze

Question
I have a 1997 National Seebreeze with a 460 Ford with 39,000 miles. After driving at highway speeds for awhile and letting up on the gas the engine sounds like it's reving looking at the tach does not confirm this. Giving it gas while this sound is happening it will not excelerate. it only lasts for 10 to 15 seconds and doesn't happen all the time. it just concerns me. Any ideas?  

Answer
Hi Edward:
Have to admit that saounds strange. I would have to say that if the tach does not show an increase you do not have one. Given that the tach works ok otherwise. Couple of things could make that sound. You might be activating the air pump somehow and it is sounding different when you let off the gas. The anti backfire valve could also be activating when you let off. The fan clutch may not be working right, need to check. The fan clutch should spin very freely when the engine is cold and stiffen up as it warms up. The transmission could be shifting out of overdrive when you let up, or the torque converter comes out of lock-up. The fact that you can't accelerate then seems to suggest the the transmission is freewheeling. Try and find out if the problem is speed related by letting off the gas at various speeds.  Thats all I have. Good luck.
Bill