You would need to take down the under belly and measure between the frame rails and cross beams to get an idea of the size tank you can add. Then order the tanks along with 4 bulkhead fittings and appropriate size hose to join the new tank to the old. Joining the tanks (old grey to the new grey) will eliminate any other kind of headaches with plumbing. You may also need to bore a hole through a cross beam to join the tanks via hose.
Or just put in a valve before your water pump to switch between auxillary tank or original tank to draw fresh water from.
Rather simple really. Black water, I would not mess with.
*Current: 2005 Ford F350 Crew Cab Dually 6.0 diesel 4x4*
*Retired: 1987 F350 Crew Cab Dually 6.9 turbo diesel
2007 Forest River WildCat 32QBBS