It seems like the right order to do things actually, I can understand starting off that way. It's not too different from simply building a traditional house on the land permanently (whatever you think of building on your parents property), both are a step up in certain ways compared to simply living in the parents house. And this one is on wheels, it can be moved away at some point if they want to.

I think it'd be worth it, if at all possible, to build in such a way that real plumbing can be swapped in when the time comes. "litter box for humans" is making me cringe :)

Also the parts of this story that are about them as a couple are cute I think.

You're begrudging them their accomplishment because they are currently using her parent's land and electricity?

Hacking together their tiny house on a shoestring budget and documenting the endeavor for others hardly deserve such dismissive disregard.