2720SL-330.00 labor to add water tank...

:-*OUCH--on this 2720SL we are in process of buying(you may remember recent tire thread) we were telling our dealer that some of you guys had put in a secong fresh water tank under the sink, i believe. well, the dealer called jimmy davis about it, and was told that he would have to remove the water heater and one of the other  appliances to take out the old tank and replace it with a new 20 GAL. tank!! tell me again that this is unnecessary--that a separate 11 gal tank can be added. or do we need to pay so much and have a completely new tank (one 20 gal tank) installed at 330.00 plus cost of tank and shipping. meanwhile, i'm going to try to find the threads on this board that talked about adding a tank to this model. thanks so much again!  
2swans