SURGING PROBLEM

I have a 2007 Winnebago Adventurer Model 35L with a Workhorse Chassis with a 8.1L Chevy engine and a 1000 model six speed Allison Transmission. While running on level terrain at about 60 to 65 mph the tachometer starts surging from 2,000 rpm up to 2,500 rpm as if the transmission is trying to shift but never does. I may have driven anywhere from 25 to 30 miles before it starts surging. The surging can go on for 2 miles or as many as 100 miles then stops and goes back to normal operation. I have had the Transmission rebuilt, replaced the ECM module and replaced the spark plugs and wires. Additionally, I have taken my coach to Allison’s main district area shop in Houston, Texas, and other locations. I was told that without the Check Engine Light or a Code indicating that there is a problem, that this issue cannot be diagnosed and therefore the problem is unable to be repaired. I would welcome and greatly appreciate any ideas or solutions that may provide a “fix” for this problem.