For feasibility, all it takes is a plan, a decent cabinet shop, and money.
As for cost, it has typically been a $2000-3000 option where I've seen it, and a good part of that is going into the electronic equipment.
Since you are doing it after the fact, the choice of equipment is yours. The rest will depend on choice of materials, complexity of the design, and how fancy the cabinet work. Rustic finishes, plain designs, low cost woods can save money on materials and shop time, exotic woods and intricacy can push costs to the sky.
I don't know that Winnebago has done a trundle twin at the Minnie/Access/Spirit trim level, but if they have, the parts may be readily available from the factory, ordered through a dealer. I do know they had an entertainment center option for the Spirit, so there will have been OEM cabinetry matching the decor and engineered to fit the Winnebago cab-over.
Also be aware that a trundle twin will not really be a twin. In order for the bed to stow away under the cabinetry, width will no more that half the 57" depth of the overhead space, most likely 28" wide, about a foot less than a twin, a couple inches less than the 30" of what is called in Europe a "cot" or "small single." A space that works for a small child or thin, quiet adult, someone who can comfortable sleep on a sofa seat maybe. A friend of ours had this arrangement in an Outlook, there was nobody in the family that could sleep up there, even the grandchildren were too large.
On a custom build, you could make more shallow cabinets for the entertainment center (most equipment will now fit into 18-24") and leave 33-39" for the bed (twin mattress is 39") but the bed would no longer stow entirely under the cabinets.
Tom Test
Itasca Spirit 29B
