Dog smell that is not urine based is normally from coat oils, hunting and water dogs are the worst as they are designed with heavy coat oils to repel water, especially Labs. If the owners bathe the dog frequently it is not normally a problem, but few do.
You need a solvent/detergent which breaks down organic oils. I used to use a Taski product (Degrease) -
http://catalog.highpointss.com/catalogp ... 97368CCASE
years ago that was made for cleaning restaurants and meat packing facilities that was wonderful at eliminating dog oils (and their odor). I don't even know if it has changed formula, or if there is now something better, but it used to be the bomb.
Pay special attention to areas the dog usually laid. You ca tell because it will leave black, rub marks on adjacent walls and furniture.
Also, something frequently overlooked - wool carpet smells like dog when damp. Some folks don't even realize they have wool carpet and don't understand why they can't get the "dog smell" out of their carpet.
PS- There is little need to fear bringing a disease or infection into the next home. The vast majority of canine diseases and parasites are species specific and cannot infect humans. They also cannot live long off of, or out of, the host. You should be much more worried about infections picked up from humans, than pets. However even human diseases normally do not exist long off the body.
Take care,
lisa