I figured it out!

 

I know what’s wrong with the motorhome.

 

First the engine blew up.  Then it got fixed.  But on our first test drive, the rig seemed a little bumpy.  It felt like it was running rough at 65mph.  I took it back to Josh at MHC Kenworth.

 

Josh checked it out and everything seemed fine while it was idling.  He plugged a computer in behind the dash and monitored all the engine and transmission functions while we went on a test drive together.  There were no error messages, no readings out of range, but still it ran rough at 65mph under load.  He reset the transmission.  We drove the motorhome away to see if resetting the transmission helped.  It didn’t.

 

The motorhome ran terrible at 65mph in 6^th gear, there was a shudder, but if I downshifted, it got better.  It hates 1,500rpm.  It hates 60% to 80% load.  It’s fine with 1,800rpm.  We took it back.  Josh plugged a computer into the outside cabinet under the driver’s window and ran a program to reset the timing.  He plugged the computer into a fitting at the right rear of the coach and updated the engine software.  He checked the exhaust brake to make sure it wasn’t sticking.  Nothing helped.  We drove away.

 

Josh is stumped.  Everyone at MHC Kenworth is stumped.  Caterpillar Tech Support is stumped.  Allison Transmission is stumped.  We’ve driven a thousand miles down the road in 5^th gear because it’s too rough in 6^th gear.  When it gets too rough in 5^th gear on an incline, I downshift to 4^th.  Suddenly it comes to me.  I know what the problem is!

 

I’ve listed every clue.  I’ll leave this as today’s puzzler.  I’ll send the answer tomorrow.