[prairieschooner]

Our '82 was reportedly upgraded at Newell during a previous ownership. That is when the Holding Tank was upgraded to a single Tank with Valves that allow the Gary Water to go Overboard or into the single Tank. If I was able to bring our coach to my cabin I would not hesitate to use the Gray Water to water the Forest, actually most of the people that live up here Full Time have done this with their Cabin's Plumbing.
I am not sure about the individual capacity of the black/gray tanks so I cannot comment. I agree that removing the water tank first would be allot easier to gain access to the Holding Tank. Repairs to that Tank would then need to be evaluated after an examination of the problem. With all of the work involved I may consider a new tank just too be safe.

After you remove the Holding Tank may I suggest that you support the Tank and fill it with Freshwater (maybe flush it first) and use Food Coloring to help find the Leaks. Red or Blue worked the best when I was doing Boat Repairs, I stayed away from Yellow or Green for obvious reasons.