My 2008 Elite had a bank of switches just inside the door that enable different parts of the DC system. One is labeled Water Pump. Unless it is flipped up to the "On" position, the On-Off rocker switch in the galley cannot turn on the pump.
Does your 2018 Elite have the DC system enabling switches?
Also, do you have a rocker switch outside, near the exterior shower hose, so you can turn the pump on from outside? If so, make sure it is firmly in the on or off position, If it is at a midpoint "null" position, you will not be able to turn on the pump from the inside.