This forum section seems to have the people that run heavier weights and so will probably have the experience that I need to draw upon. I am getting ready to select the transmission for an LBZ (have engine) installation into my rear engined (currently a gas 440) motor home.

If I use a 5 spd, the rpm at 65mph will be about 2200 rpm.

If I use a 6 spd, the rpm at 65mph will be about 1900 rpm.

The actual GCVW will range from 14K to 18K 100% of the time.

I prefer to lean towards economy vs power but not to the extreme. I want to select the Allison transmission that will be best for the Dmax over the long pull. My subjective preference at this time is the 6 spd with the additional overdrive to bring the rpm down a little. This should help mpg, extend engine life and reduce sound levels as well if my uninformed reasoning is correct.

My concern is that I don't want to bring it down too low and cause engine problems over time. Is a Dmax "happy" at 1900 rpm on level or slight rolling hills at the above gross weight or does it need a few more rpm to live happily ever after?

I don't want to make as important of a decision as this based on a coin toss due to my lack of knowledge.

I thank you for your input. I have done a lot of reading so feel free to write long answers!