My work had a bunch of Sprinter vans. At around 100,000 to 112,000 miles the transmissions failed. They had to replace the transmissions, as the local dealership would not rebuild them. Stating that there is nothing to rebuild, a new transmission they could warranty, not a rebuilt one.
Turn around time for our vans was about a week at the most, as the local dealership had them in stock already. It really only takes a few hours to remove one and install another.
As for your tank, you can check this place.
Tank-Depot.comThey might have any fittings that you might need, and have various tanks in stock too. You might consider a second tank, or install a system where it will be easy to fill your replacement tank from inside (or outside) by pouring in a 5 gallon tank such as the blue water totes sold at Wal Mart for about $8. So once your fresh water tank is down to 1/2 full, refill with a couple of portable tanks?
This can also be done with a winterizing kit, and a separate input hose, just draft all the water from your portable tank into the fresh water pump, and leave the fresh water fill valve open between pump output and fresh water tank?
Good luck,
Fred.