I have no comment on dealer websites as many are good and many are bad.
As for manufactures websites, I doubt you can beat Winnebago's. Not only can you see every model that is currently being built along with downloadable brochures, diagrams, parts/service information, you can also obtain everything (and I means almost everything from air conditioner down to the type of screw used to hold the handles to the cabinets, all wiring and plumbing diagrams along with colors, material, etc) used in your motorhome going back to the 1960's. I like to look at the evolution of a particular motorhome from it's initial introduction in the 70's to the current 2019 model. (not to mention the way the sales material went from drawings to photos).
From a sales perspective, the Winnebago site is on par with all others. From a owner's, historical and maintenance standpoint, it blows all others away!
1994 Itasca SunDancer 21RB - Chevy G-30 chassis.