Transmission question

[WE B RVN]

Hello All
I have a transmission question. I have an Allison 6 speed behind a Cummins 300hp turbo. The engin rpm at low speeds has started dropping to low before shifting. For example if I slow for a stop light and the light turns green and I do not need to stop. Or if I'm cruising along a back road and start up a grade. The rpm's will drop below 1500, as low as 1200. This is at halve throttle. To get it to shift I must push the throttle on down to the floor or shift it manually. At 1500rpm at halve throttle it starts to smoke. As the rpm's drop below 1500 and I'm pushing the throttle down, the smoke gets quite heavy. It is somewhat embarrassing. I serviced the engin last summer, replacing all filters including air filter. I have not serviced the transmission. I have just over 50.000 miles on the coach. This all started at the same time I had to switch to ultra low sulfur diesel fuel. I do not think this has anything to do with it, but thought I should pass on that info. I first thought that the program parameters needed reset. but the transmission preforms flawlesly other than this problem. So now I'm thinking that the transmission filter may be clogging and dropping the internal fluid pressure confusing the computer. I'm by no means a mechanic. However I have driven diesel bus's for 28 years. Our fleet mechanic's will not touch our transmissions before 100.000 miles. I was thinking that I would change the trans filter without changing the fluid, this is much less of a mess and I can do that myself. Your thoughts on this would be helpful. I would like to resolve this issue myself without taking it to Freightliner.
Thanks Kevin