Leaking bathroom sink drain

We have a 2011 Holiday Rambler Vacationer. On a month long trip to Colorado and just discovered the bathroom sink drain pipe is leaking, somewhere near the exterior wall beside/behind the hot water heater. We have gained access to all the pipework by taking out all we can under the sink. Anyone know the next step? My husband assumes that the hot water tank has to be removed to get to where the pipe is leaking. Anyone with experience with all this? We can live without the bathroom sink til we get home. Just wondering what is involved. Thanks