If you were considering using a FRP body go look at a Penske, Ryder rental truck. They use a FRP body and then inside the FRP they use a Z metal stud. Attached to the Z metal, running horizontal, are woods runners, probably hardwood 1" x 4".

This does create a space for insulation and this style of construction would support the cabinets. Also a cavity for wiring. Wiring cound be run at the top and bottom of the walls using a 45 degree panel to cover it. This is done in some RV construction. One advantage would be by removing the panel you could service, change, the wiring.

In some ways the steel framed body mfrs, Showhauler, NRC and United Specialities, are similar except on the inside of their steel wall studs they put OSB panels. They then glue a metal skin on the outside of their steel studs. The Z studs would be better for insualtion because there would be less gaps in the walls that are not insulated. Tubing creates a air space with no insulation.

The appearance of using a dry van body would be similar to a Renegade. Check with the manufacturer of the body to see if they offer a FRP with a smoother exterior finish. Some of the FRP panels have a rough texture.

FRP is heavy and adding interior studs will be more weight. But steel construction is not light either. You could probably find aluminum for the Z studs. Or you could use aluminum C channel also.