Hey check out my site. I have a design that would work out for you. It is called the "Micro Habitat"
It is only 10.5' long and 6" high. It could eaily be scaled up a bit for your added height. It is a pretty straight-forward design so building it from wood is pretty easy. Using the sandwich method I mentioned earlier you could make walls only 1" thick and still have plenty of structural strength (I ran the idea past my brother-in-law, who is an engineering student and he agrees).
The bathroom could easily be fitted with an RV shower enclosure and would be an all in one unit.
The unit has A/C and interior kitchen, and I could add anything else you wanted. It is a Tiny Travel Trailer rather than a tear.
You could also make it a lift up wall like another of my designs and have the best of all worlds. Heck go nuts with it, mount an LCD or plasma TV and a DirecTV dish.
Do not worry too much about the welding. You can usually pick up an old beat up utility trailer for pretty cheap. I just saw an old busted up pop-up tent trailer down the street from me for sale for $300.00. The canvas is gone and the top has trouble staying up, but it has lots of great parts.
The utility trailer I bought for myself I found on Ebay for only $5.00 over in Mesa.
Let me know if you have any questions, or would like a custom design. I work ver slowly for free, and pretty quick when there is money involved
I could even make you a great pdf file with the patterns for the sides which you could take to a print shop and have printed full size.