Tamsun the reasons that your Cat or indeed any Cat's are interested in Birds is a pent up frustration in not being able to pounce on them because she is being restricted by the confines of the motorhome or the leash.
The tail twitching is a sign in itself of aggression with prey or indeed you for harnessing it. You will never control these urges and instinct's.
That being said you are also being responsible in dealing with the issues and with that I wish you and your Cat success in your challenges and life.

Thanks so much for your feedback. I totally understand animal instinct being part of their nature and used to feel bad about not allowing her to hunt. I guess with the world the way it is evolving my conclusion was to ensure her safety out in the wild as well as the local wildlife. Do you have cats? Any other tips?

Tethering a cat outside is just asking for trouble. How will it defend itself or escape from a roaming dog

I completely agree which is why I have only tried my cat with limited to no people or animals around whilst constantly watching her.