You may have to look at the used C market. I'm not aware of C toy haulers being built today.
FourWinds once had a FunMover line, smaller models (to 32') on E-450, larger on F-series chassis. I do not see equivalents to the FunMover in today's Thor Motor Coach line. Jayco had a couple toy hauler models they built on Kodiak chassis, would have been in the 32-38 foot range.
Forest River has a commercial division, builds a Rockport box truck. They also own Dynamax, which has a specialty vehicle division, but as far as I know they have not built toy haulers. I would approach a truck body builder about customization. A builder associated with a RV manufacturer would have access to RV parts, maybe design and building skill.
A 25 foot C is going to have a box length of about 16 feet. Use 10 feet of that for a garage, you have six feet left for the house, which is why toy hauler motorhomes, back when they did make them, tended to be 30 foot or longer.