We just went through there last month. You can get in and out of the park without going through the tunnel. We went through the tunnel because it was easier the way we were traveling. Here's a picture of the "Tunnel Procedures" they gave us. We sat at the entrance for awhile while they let cars go through then we went right down the middle of the tunnel. The length restrictions are because of the 7-8 sharp switchbacks going up to (or down from) the tunnel.
Watchman Campground in Zion National Park is very nice. Two loops have electric for $18 ($9 if you have the Senior or Access pass). There is a dump station in the campground and water spigots throughout. Most sites are level. You can get a site in the trees or in the sun, on the river or not. You can walk to the visitor center. I think the shuttles have stopped for the winter, they were running weekends only when we were there.
Enjoy
Wendy
Calico Ghost Town, Yermo, CA