I think you may do best to run the pump off a car battery maintained by a trickle charger.
A six volt car battery was used to power the radio set.
Low-voltage lighting systems use a transformer to drop the 120-volt house current to 12 volts, about the amount of electricity used for a train set or car battery.
The battery in an integrated backup battery machine is also lighter-weight than other backup systems, as most CPAP machines require 12-volt batteries. 12-volt batteries are about the same size and weight as a car battery.