Leaking Coolant when Towing

I have noticed the last few times I have towed that coolant is leaking out the degas bottle at the cap. The dash temperature gauge hasn't changed and it doesn't seem to run wierd or anything. I don't smell fuel in the coolant so I don't think its a injector cup or anything like that.

Do I have a bad cap or thermostat? It only seems to be a problem when towing, never does it when unloaded.

Thanks

Brian