I built my own about 10 years ago and still enjoy it. About 2 sheets of plywood, 2 pair of slides, 8 hinges and a pair of locking paddle handles. Sprayed it with Rhinoliner and done. Lay the second row down, put 2x4 block under it to level seat and you have a sleeping platform.
Actually I'm gonna disagree, the trans are fine it is the programming that causes the problem which is usually torque convertor failure. If you get a good tuner that problem can be easily solved.
Most of the time I get high 13 low 14 mpg with a built 6.0 and 6L80 trans. Can get better if we tune it for premium fuel. Highly recommend 6 speed transmission