New Horizons custom builds fifth wheel RVs in a range of sizes. They could probably build whatever you would like to design. I don't think anything in your requirements really requires a garage and loading ramp, which is what makes a toy hauler a toy hauler, except you did specify a ramp. But I suspect New Horizons price list is way over your cost expectations.
I understand the toy hauler recommendations because they come in a range of prices and have an empty space in addition to the house facilities (usually up front).
You might also consider motorized if constantly moving. Winnebago and Forest River both have divisions that do custom builds for non-RV applications (mobile offices, command posts, bookmobiles, mobile stores, medical facilities, etc). Any mix of storage, office and living space is feasible, provided the coach is sized to fit the need. Again, this is not cheap.
$10,000 today in the new RV market will not buy a small travel trailer. I suspect for a toy hauler large enough for your list of what you want, you'll be closer to $20K before going to somebody for customization.
Tom Test
Itasca Spirit 29B