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If this has multiple slides then you need at least two series 27 batteries or better yet two golf cart 6 VDC batteries hooked up in series to provide the power needed to keep the pump pressure up. Keep the shore cord plugged in or the tow vehicle running when you run the slides. It sounds like the voltage is dropping due to having just one battery and the pump isn't putting out enough pressure to move the slides out with the resistance they have. Wiper seal lubricant would help as well. Please get back to me if you need more on this or hooking up the batteries. Perhaps a phone chat would help as well.
Were you able to correct this with multiple batteries? Trojan is one of the best, ***** ***** Interstates are about next best.