OK. So you have enough money to pay for the trailer outright. You want a small space (but that will store your dirtbikes and car). You want as little social interaction as possible. You don't want to go into the "red zone" money-wise. You want to be able to modify things inside. You want security for your things.
RV Parks are not known for solitude. Because all the RVs are small, many if not most folks will be outside (which is very close to your RV walls) much of the time. So, you may find you have to leave your home just to get that mental distance you need from other people.
You can only modify so much inside an RV - 200 square feet doesn't provide a lot of options to install a theater speaker system and a nice flat screen TV. Often there is one place the TV can go and it's not convenient for spending days on end watching or playing video games. Not a lot of floor space to set up the computer and media server. You'll end up having to do double duty with space. Remember, you have a lot more versatility with an empty box (residential room) of 200 sq ft, than you do with a 200 sq ft space pre-filled with built in furniture, cupboards, utilities, etc. - so the fact that you can happily fit all your stuff into one room at your parents' house doesn't equate to being able to fit it all into an RV (trust me, I tried years ago and it did NOT work well - just trying to get a pair of pants out for work became a chore of moving things around).
In addition, the trailer will depreciate. So whatever you put into it will be gone. And the more you strip the inside to fit things, the less you'll get for it when you're ready to move on.
Have you looked at mobile homes? If you're willing to expand your potential home area, you might find one that you can buy for little down. If it's got more room than you need, then just close the doors to the rooms you don't use (but you'll have them in case of a layoff, etc.). When you want something better, you won't have lost money in the purchase - mobile homes don't go up in price as quickly and traditional stick & bricks, but they don't depreciate like an RV.
Have you looked in Petaluma, Cotati, Rohnert Park? When I got my first place, I ended up in an old (1900) Victorian 2-story with basement converted into 3 apartments. It wasn't the nicest place in the world, but I had a lot more freedom in that place than anywhere else - I wired speakers throughout the apartment, I painted walls whatever color I wanted, I had whatever pets I wanted, I installed cupboards on the walls, I tore out carpeting, etc. As long as I paid my rent, the landlord pretty much let me do anything. Look hard enough and you can find these places.