vreihen,
Mine came with no side door. I wanted to put in a real RV door with screen and deadbolt. I got one and had to cut off almost a foot from the bottom to get it to fit. It turns out that all the model 1610s come with the door pre-framed, as I found out when I pulled off the original waferboard (OSB) on the inside. The frame was slightly oversize for the RV door, so I shimed it from the inside with wood.
Your door is a little wider, fitting the framed-in door opening exactly. It is (most likely) a solid waferboard (OSB) door. You could build yourself a new door that's insulated, or you could glue on some foam insulation and perimeter wood to the inner side and panel over it. You can put in any kind of small RV window that will fit, depending on the depth of the trim ring.
New RV doors with screens are extremely expensive these days. I got mine for free from a buddy who took it out of a nearly-new motorhome (on an insurance claim) because the bottom was dented and punctured. That was the section I needed to cut off anyway.
Good
Prem
My goal...
_____________________________________________
...is to live in a trailer.