It's true for all vehicles. I haven't found a max height allowed on a ferry, but it's higher than a semi truck
Upper decks are 9', lower decks are probably 20', if I recall correctly. They're making up the difference with increased per-foot costs for vehicles over 20' in length. Good news for campervans! Bad news for tent trailers
And I could care less about 40' diesel 'rolling resorts' and giant 5th wheels!
The Anacortes, WA-Sidney, BC (near Victoria) ferry operated by WA State Ferries has also done away with overheight fees for RVs. BUT they still list an overheight fee for regular vehicles 20' and under, and all RV fees start at 20 to 30 feet... so not sure if you'd be stuck paying in a van that's less than 20 feet (RB only, I think?)
Given the choice, take the BC Ferries anyway, and cross the border at a land crossing - much nicer ferries, and the WA ferry customs process is slow. Coming Canada to US, you do customs twice (!) and there are only a couple of lanes. I've heard it can take upwards of an hour to get out of there. I rode my motorcycle and was first on, first off, so no wait
Peace Arch crossing has been ~5 minutes for me each time I cross, in the last couple of years (though I tend to cross during the week, or early in the morning).