I'd basically agree with most of Greg's suggestions - although the last two would be tough. I don't think ya need to get a new Sprinter and if ya decide that sleeping in air that's below 60 degrees works for you, ask them to give the pneumonia vaccine when ya get your flu shot ..... me, I couldn't do it myself.
Another thing to consider is getting a short tailpipe extension - to make sure that the exhaust is actually venting away from the underside of the vehicle.
Reason why: all along the inside lower body edge, there are little slits that are approximately 1/2" wide x 1" long - these are condensation drain holes - and they the cavity that they are draining goes all the way up into the interior of the vehicle, unobstructed by anything. Unless you seal these off - or have significant positive airflow into the Sprinter (check that cabin air filter to make sure it's not clogged up) these drain holes can allow outside air to come in. With warm exhaust, simply by convection.
I've idled mine quite a few times overnight - and I have a working CO dectector - I did have it go off at one point - but it was not while I was sitting idling, interestingly.
Getting the Espar is probably a good investment if you can swing it - more economical in the long run, saves wear and tear on your engine, and is vented to the outside. I just had one put in myself up at Espar of MI.
And Marty's suggestion seems like it would be worthwhile to explore.