new leaf wrote:Does anyone know what come 1st is it the trailer or the pod that sits on the trailer

most of us start with the trailer, unless you are a pro and can draw it all out and figure material weights ahead of time. You need to know the trailer dimensions then figure out how to connect the tear to the trailer. Even if you go the custom trailer route, you need to have a good idea of door and axle placement. A door forced to be out of the normal spot you sit in your bed at home, will cause aggravation, getting in and out of bed.
The other big issue is cabin width, 4 or 5 feet wide being the most common, with 5 the big favorite here. Sitting a 5 wide on a 4 wide trailer adds issues ( easy to overcome, mine is built this way) it is cheaper to get a used trailer ( old pop ups are a great source) then you know what you are building to fit, and you have a stable, portable, build area. ( getting a tear off or on the trailer is a pain, I did it solo, it wasn't easy!)