John, just as a suggestion if you don't already have it: You might want to also install a reversible, multi-speed fan in the front roof section of your rig.
With your new A/C running in the rear area (where it is?), turn on this fan and set it to blow out. This method should then help to draw/move the A/C cooled air throughout the interior.
We do the above a lot with our roof fan in the front roof area above the cab so as to draw air all throughout the coach's interior.
We don't have to do this when running the stock A/C on the roof because it has ceiling vents throughout the coach's interior that are adjustable. However we do have to tolerate it's noise and maybe also the generator's noise when doing this, depending upon whether hookup camping or drycamping.
2005 E450 Itasca 24V Class C