True, manufacturers have a handful of basic plans they work from, but rarely will two identical houses pass each other on the highway. Personalization is a trademark of the industry. Rarely is a request turned down if its legal and you have the means to pay for it. Each manufacturer has their own limit, or lack of a limit , as to what they'll throw on, so it's up to you to call around. They sell numerous unfinished shelters, which consist of just the bare bones trailer and shell, leaving interior design up to the purchaser.
Most heating systems are propane fueled and thermostatically controlled.
Again, if you have a specific idea in mind, share it with the builder upon placing an order. Fully automated houses even offer rattle-reels, automatic jigging devices, and lights directly above the holes. I'll raise you a livewell.