For at least 20 years, Econoline then E-Series have dominated fleet sales of full-size vans and especially sales of cab-chassis or cutaway series to aftermarket vehicle builders.
Chevrolet had a small window of leadership when first introduced as the alternative to Dodge, but lost it in the late 70s when the 3rd generation Econoline was introduced with body on frame replacing unit body vans. Chevrolet should have made some gains when the body on frame Express was introduced, but the market is the final vehicle builder, not the end consumer, so commercial arrangements take precedence over brand preferences of the final buyer.
Ford got a big jump ahead with the introduction of 4th generation, now called E-series, with new engines replacing their 1960s engines, while Chevrolet and Dodge were still using engines and transmissions dating back to the 1950s.
Chevrolet's chance is now, they may be the last man standing, unless they decide to replace the product line before Ford stops making E-series chassis.
When shopping recently for a 1-ton 12 passenger van, I thought I would really prefer the Chevy over the Ford, the specs all look better. But inspecting build quality, finish details, and driving both, I ended up buying the E-350 instead of an Express 3500.