Power cuts out when pump runs (on DC)

I think you probably have a poor connection somewhere between the battery and the DC distribution panel, quite possibly at the main self-resetting circuit breaker for the battery (or possibly inside that circuit breaker). A poor connection there would not only cause a voltage drop, but would also make the circuit breaker warm up a good bit and so trip at well below its rated current. That said, there may be a poor connection in many places, and it could be on either the positive or the ground side. Go through and check that all the joints are clean and tight, and the wires have not corroded away (particularly those attaching to the battery).