As I recall (I sold my '94 LY last year), the engine oil capacity was about 15 quarts as the sump for the air compressor is included. I can't recall the tranny capacity (I did have it serviced once at an Allison shop where the mineral oil was changed for synthetic). The propane tank is 80 lbs, fresh water 80 gallons, wash water & waste are 40 gallons each.
Diesels can go 10,000 miles between oil changes. However, if you use a premium synthetic oil, you can go longer intervals, subject to oil analysis, which is what I am now doing with my Dodge Ram 2500 (the PO had been using Amsoil and I've continued doing the same - I sample the oil and send it in. I've done 23,000 miles on it and it is still good, but I'll change the filters at 25,000 miles.).
One thing I will recommend though, is the Steer Safe system. It makes a world of difference in the handling. Here is their website: Steer Safe