Answer:
This is a mountain phrase that means "within hailing distance," which may be a mile or more away. It comes from the mountaineers' way of yodelling or whooping to each other from one height to another, once a common feature of the Swiss Alps and the Southern Appalachians.
Added: Agree with above answer - additionally - the "holler" referred to is actually the common idiomatic pronunciation of the word "hollow," meaning 'shallow mountain valley.' Thus; "Only a shout and valley away."