I'll take a stab at this, even though I have a gasser and no DP personal experience. :)
When I think of "maintenance", it's overall maintenance for the entire vehicle, not just the engine. We can debate costs of oil vs. quantity vs. change interval all day. It's probably gonna be a wash, especially if you do the maintenance yourself.
That said, in my observation when purchasing our motorhome, the DPs started about $30k MSRP above most top-of-the-line gassers. This added cost isn't just because of the diesel engine. It's (usually) a heavier chassis, which correspondingly will have larger and heavier components, which usually translates to more expensive to replace. And often requires special, heavy duty tools that the average Do-It-Yourselfer might not have. It most likely has a much smoother riding suspension than the gasser (air suspension) and usually most of the interior accoutrements are of a higher quality as well.
All of this translates to it potentially costing more for the upkeep when something needs to be replaced. That said, if you only keep your rig for a few years and then trade, then the DP might be cheaper to maintain as it's less likely to have a failure during that short interval due to the higher quality construction & components. The next owner of the rig, or the owner who keeps their rig for a much longer time, might have a different "maintenance cost" experience.
But comparisons including everything are tough because "quality" and "RV construction" are sometimes difficult to use in the same sentence, due to the great variability amongst manufacturers, and even product lines within the same manufacturer.
I'm happy with my gasser. Would I want a DP? To be honest, I really don't know. I DO KNOW wifey doesn't like the smell of diesel, so should I ever have to make the choice between a new gasser or new DP, I might also have to make a choice between keeping the current wife and getting a new one. In which case, I'm pretty sure, any additional cost of the DP will pale in significance to the cost of a new wife! LOL :)
~Rick
