Tom is not incorrect in advancing the idea that in specifying the 5.4 litre V8, one might be getting “lesser” capacities or capabilities elsewhere. And it would appear as regards the commercial chassis, that would be so. But other than for towing purposes, are likely not so with the motorhome chassis otherwise.
We already know that PC (and most other manufacturers) modify stock chassis lengths. And if you look to the motorhome chassis spec sheets, you’ll divine that in order to get the max GVWR rating and fuel tank capacity for all E350-based chassis models, whatever the final wheelbase; PC likely orders the longer chassis and cuts them down. It would seem alternator ratings would likely be the same for a motorhome chassis across the entire line, and that the 55-gallon tank comes standard on that longer chassis as well.
On the other hand, the 2100 would fit on a standard Ford Chassis without modification from a longer chassis. But if that were so, AND with the 5.4 specifically, the GVWR would be less than as advertised by PC, as well as lesser standard fuel capacity per Ford vs. PC specs. But that could be made up by special-order GVWR specs by PC, as well as an optional fuel tank size. It’s tricky and inscrutable.
In the end, just go with PC. While it would seem you could save $1,200, some of that savings could be eaten up with chassis modification and option costs on the downside. A smaller motor, forced to work harder, may only end up matching V10 fuel economy anyway. And there are likely myriad other considerations for PC in manufacturing a house on a V8 vs. a V10 chassis on a lot of levels. I’d say stick with the “standard” V10 PC chassis and be done with it. Every change, while there may be a corresponding savings on the front end, could have additional costs on the back end.