I have 2 Mauck MSV motorcoaches (google them, very rare and trick) they are both 1998 models with 454 gas pushers and mated to Allison auto transaxles. How possible is this as the 454 is getting me around 8mpg and they are a bit short on power. I know the Allison will take the torque as well as the full legnth stainless frame that these motorcoaches were built with. There is plenty of space in the rear engine compartment to put a Duramax but will it mate to the Allison transaxle and how much of a pain in the a$$ is it giong to be. I have a H1 Alpha and that goes like all hell with no mods to the motor or computer programming. So, tell me o gurus of oil burners, is this a possibility or a dream??  

Depends on if the model of Allison tranny that's on the 454 is the same bellhousing pattern as the Dmax. The other issue would be whether or not the electronics are compatible. Sounds possibly easier than some other swaps, but I'd want to do my homework first.