You probably have two batteries on the Dolphin with the workhorse chassis. 2005 figuring they have been changed out at five years, it probably needs two new batteries again. But load test them and they can tell you if the batteries are weak and need replacement.
There is no reason why anybody has corrosion problems. It is lack of maintenance on your part because everybody that owns a RV should wash the batteries at least every two months. You will never have any corrosion problems if you do that.
Sorry! I usually do not read all the posts, only the original. So I see now that you did find the problem. I am glad for you that the problem has been solved.