The manuals for Travel trailers are generic. They will not include specific locations or diagrams for much.
Owners manual packages are usually a collection of specific manuals for items in your trailer. ie; a manual for the fridge, another for the furnace, etc. Those individual item manuals can often be found from whomever built that item be it Dometic , Atwood, or whoever.
As to finding out which tank is which, follow the lines from the drain valves, black and grey have the big dump lines and valves which should be marked, fresh wont.
Good luck.
