Finding Wall Studs in rear of Travel Trailer

I want to install a ladder on the back of my Northwoods trailer but have run into difficulties. I can find studs with my stud finder but I know there is electrical and plumbing in the back wall. I contacted Northwoods for wall schematics but they sent me a 16 year old electrical layout drawing that is totally different and utterly useless to me. I politely asked again for a blueprint or wall schematic to help me but it looks like I don't even deserve a reply to my emails anymore. It's too bad when you think you are buying a quality built product with good customer service only to find out they are all about initial sales and then you are on your own.