On google maps, the street view and satellite view both show a couple of pretty big campers parked up at the road end parking, based on that I'd say "yes".
Pretty sure lorries were going up there when we were last there, was a few years ago but I can't see the tunnel changing in that time, we were in a 5.5m by just under 3m high landrover camper and I don't remember the tunnel being that tight, there are a couple of tight hairpins after it though.
I have sent groups up there in 28 seater coaches and they have no issues, fits through the tunnel and drops of at PMC at the end of the road.
I cant give you an exact size but I think the coach would be bigger than your van.
There are a few local coach companies, you could always drop them a email. I am sure they could give you exact sizes as they have to go to all these places.
It is a tight turn, but easily doable in a camper van. The main prob is meeting traffic coming in the opposite direction. It often gets a bit backed up there. As a result, I've always made my way up in the quite of the late evening or early morning..
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