I have two 6v batteries so shouldn't be that
You do NOT know the exact status of your batteries charge if you do not own a cheap electrical meter that will measure voltage...
Harbor Freight or Walmart or any hardware store will have a meter for under $10 that you can use to measure the exact voltage of the batteries.
With the electrical cord unplugged from shore power...
Measure using the VOLTS function on the meter... Red lead to + battery terminal and black to - battery terminal
Each 6V battery should read at least 6.3 volts (12.6 volts from + of one to - of the other battery) with no load (nothing running on 12V). Then while running the pump
, measure the voltage again and see if voltage drops from the no load voltage. Any drop more than 0.1 or 0.2 volts shows weak batteries.
If there is no voltage drop yet still have the DVD problem, then I would figure on a bad connection to the 12V line that the DVD works from.
I am going to bet that your batteries are weak, and the voltage drops below 12 V when the pump runs, which interrupts the computer that runs the DVD.