I bought a polytunnel three years ago, salad leaves, beetroot, cucumbers spinach, rocket all grow well, but tomatoes just go leggy and get fungus, fruit rots before it gets big enough to pick, tried all types, yet the same plants outdoors do very well. is it something to do with the polythene?
There's probably not enough light getting through the polythene & it sounds that you have a ventilation problem causing blight on your toms. I had exactly the same problem in my polytunnel last year.