Hi Everyone and thank you for contributing.

I think the suggestions make sense in planning the route with enough waypoints to 'force' any navigation device to take the route i had planned.

I think its also makes sense to buy a cheap navigation unit when I land in New Orleans. Thanks for the tip Landmariner. Plus i can reuse it on other road trips i go on in the US.

Just got some bad new this morning though as i read that holiday and a potential sky bridge between the UK & US will not be in place until the end of the year. Took all the fun out of planning the trip and now im planning a trip which looks unlikely to go ahead.