Bernese Mountain Dogs

I have a 5 year old Bernese Mountain dog. I am having a very big problem with getting her to go to the bathroom on walks. She will only go in our backyard in one specific spot. I am at wits end with ways of breaking her of this habit. I have walked her 4 miles in the morning before I let her go to the bathroom and still she holds it in until she gets back to the house. I am taking her on our first vacation to a shore house and I am afraid she will not go to the bathroom in the yard there. I don't want her holding it in so long that it makes her sick. ANY suggestions at this point would be greatly appreciated. I have owned about 10 large breed dogs and have never come across this problem before.

Thank you,
Linda