It's gonna depend. If the property had a "Beware of Dog" sign posted on it, you should have not gone onto the property without making sure that the dog was confined. Similarly, if the person who asked you to the property told you that there's a dog on the property, you should have waited. If you had no way of knowing that there was a dog on the property, you might have a case.
It's also going to depend on how bad the bite was. If you had a couple of minor wounds that could have been seen by a regular doctor during normal business hours or a doc-in-the-box, but you chose to go to the ER instead, then you won't get all of your bill paid. If you needed immediate attention (stitches and such), then a trip to the ER was justified.