I agree that ventilation would be a problem in a small tunnel. In a larger one you can open the doors at each end to get a through draft, and it's unlikely anyway that the sheep would go in in clement weather, just when it was raing or snowing.
Do your sheep have horns? I have told this tale before - we had a 4 horned Jacob wether who decided that the plants inside our tunnel looked more tasty than those outside, so he used his top horns to cut a neat triangle in the polythene cover. He and his half a dozen chums all stepped prettily through the hole, demolished all my crops, then stepped prettily back out

I'm sure they could do it from the inside too.
We have used our veggie tunnel as an emergency shelter for the occasional newly lambed ewe if the weather is atrocious. We build a hurdle pen inside (well away from the walls

) and just keep them in for a day or two. The patch where the pen has stood is extra fertile for the veggies later
I wouldn't put up a polytunnel specifically for a sheep shelter, unless it was a green mesh-covered one, as it's very simple to make a purpose-built shelter from four corner posts, some sarking and tin for the roof. You could even make one from old pallets - our first ones were like that, with the gaps helping with ventilation.