It sounds like you're just needing to activate the reverse light function on a standard 5-Way connector so your boat trailer actuator will use the lockout for you to move the trailer around by hand with your dolly. I don't know of any handheld 12V power sources but you should be able to use a small battery like what you find with a breakaway kit # 50-85-313 and then wire the power only to the last pin on a 5-Way like part # HM47910 to send power to the lockout function on your actuator.
It... view full answer...