Twins in a Casita
Liberty are 22 inches wide, maybe 23 max. The Independence beds are wider, still much less than a standard twin. As I recall, the Independence twins are 30 inches wide.
I am not aware of any molded FG trailer with 39 inch twins. Most of the molded FG trailers are short in overall length, and most "make up for this" by being narrower too! The combination of narrow and short in length means lots of compromises. But for this compromise, you get awesome durability.
You might be able to convert a 25 foot Bigfoot
. It is both the longest FG trailer currently available, and its wider too. The BF 25 has a twin option, still the twins are only 36 inches wide. The BF 25 also has a double sink and a dry bath.
If you want to play around with layout options, for a 13 foot trailer, mark out an area in your living room, 10 feet by 6 1/2 feet. Within this rectangle, you need to fit a side door, a center aisle way, kitchen, dinette, closet, and so on.
With a trailer such as my Escape
19, make the rectangle 16 feet by 6 1/2 feet.
Consider most residential bedrooms are MUCH larger than the total square feet of these trailers. And within this space, you need to fit a bathroom, kitchen, and more.
For this reason, people that want many of the comforts of home on the road often go with a large fifth wheel or a class A motor coach. I have friends with a 43 foot motor coach. I call it a "mobile condominium. It has three slide outs, a full size residential refrigerator
. It even has TWO bathrooms. When they arrive at a destination, they have all the comforts of home. The kitchen in their rig is bigger than the kitchen in my home.
With this larger size, they cannot camp in many State and National Parks. Driving down the road is more work, etc. Every RV option has its advantages and disadvantages. Molded FG is not for everyone, and my friends would NEVER consider it.