Travel trailers are dirt cheap, once they reach a certain age, and they're shitholes, once they reach a certain age. There really isn't much you can do to fix up a really old travel trailer that isn't one of the "collectible" sort (Airstream, mostly, but there are some others with collector value). They don't cost too much to heat/cool, even if poorly insulated, because they're very, very small (a couple hundred square feet). They've also got wheels, so you can drive them south when it gets cold and north when it gets warm.
And, unlike tiny houses, you can park them in most RV parks, which provides the hookups you need to live comfortably (though the older and shittier your trailer is, the fewer parks will take you, but you don't want an expensive and fancy RV park, anyway, as that defeats the purpose of living in a shithole tiny house).
I say all this only half jokingly. I live in a 32 year old Avion travel trailer (one of the collectible sorts, all aluminum and very expensive when new); not quite a shithole, but not exactly pristine, either. I genuinely recommend it for folks who have the freedom to do so.