Colin,
I have series 5 Indiana, it also gets wet in the gas locker when towed whilst its raining, i think its because of the design of the locker/front panel.
i you look at the inside front of the locker at the floor level you'll see that the front panel is not actually attached to the floor, it sort of floats if you get my drift, if your tow car is like mine, it tends to throw up a lot of spray in the wet, this is what i think is the cause of water in the front locker.
i've been assuredby Bailey's technical bods at the Feb NEC show that this is quite normal and is not detrimental to the integrity of the body, indeed i cannot find any water ingress ito the fron of the van, i've even taken out the front chest of drawers to see.