Help electrical problems! aplenlite trailer

Hey everyone I really need some help!!
My travel trailer I just recently bought a shore power isn't working. Its an aplenlite. It worked the first night we plugged it in. Then went back in a couple days later lights were dim... Batteries are charged now. New cords. New plug. House power works. Check gfis. What possibly could it be?????? Any help would be so appreciated even if we have to start all over. Owner assured us everything worked great. Tried it all out and then got home and Now were having problems. Please help. Thanks.