The previous owner of your 2008 Leisure Travel Vans Serenity could have wired the 12V wire to the reverse light pin. As such, a possible solution to your problem could in fact be to relocate that circuit to the 12V pin.
From there, you can see if the # RM-156-25 is actually supplying power to your battery.
If this doesn't solve your problem or causes any other complications, let me know and I will assist you further. view full answer...