I just bought a new gooseneck steel trailer. Very basic. Inside I want to install a bench that tripples as a step up onto the top part of the gooseneck as well as storage. My question is, how can i secure the wood to the side of the trailer without having to drill holes and run bolts through. (Wife is totally against having stuff show on the outside). I was looking into liquid nails but I dont know if it would hold up. Any Ideas?

Having just built a bench/sofa/step-up, I'm of the opinion that just Liquid Nails won't cut it.  I attached mine to an  insulated/plywood raised floor at the base and used 2x2's screwed by #10 2" stainless to secure to the aluminum frame along the drop wall.  My trailer had 3 places along the drop wall where I could do that and it does not punch holes to the outside that way.  Look under the GN to see what supports you have. 

I don't know what your trailer framing looks like or if it's all aluminum but beware, some major supports can be very thick and impossible to get a screw in to.  Like I said, I was able to do it on mine.  You'd definitely have to predrill the holes.

The easiest thing for you to do may be to just simply build a sturdy box with a lid to stick in there.