I've seen articles from both RVBusiness Magazine and the RVIA breaking down market share and/or sales by manufacturer and class but usually market share for Class A motorhomes without breaking it down into gas or diesel. However, I seem to recall an article several years ago by one of them that did show sales statistics for DPs by brand. I can't recall the details, though.
Here's a recent blurb from RVBusiness Magazine citing the SSI (Statistical Surveys Inc.) report that does cite Thor as having the biggest market share in motorhomes in general --both Class A and Class C. And I believe they also have the biggest market share in total sales of RVs overall. That's no surprise considering all the brands under their umbrella ...in addition to their own Thor Motorcoach, also included is Airstream, CrossRoads, Cruiser RV, Dutchmen, Heartland, Keystone, K-Z, Jayco, Starcraft, Entegra Coach --I'm sure I missed one in there somewhere:
SSI: Motorhome Sales Flat in Nov.; Drop 1.8%
And here's an example of an RVIA breakdown that they do every month ...these are wholesale shipments but, again, does not break it down into gas vs diesel:
RV Shipments in November 2018