I don't know if anybody can answer whether or not it's worth it besides you. From a purely financial standpoint, you will undoubtedly put more money into it than you would ever get back selling it, but that's not really your goal.
You mentioned mold on the inside; this suggests that there may have been water leaks into the framework. Depending on the construction methods, this could mean anything from extensive remediation work to a complete rebuild. Particularly if it has a wood frame, it's likely that there is rot to the structural members. Check that very carefully before starting off. I would avoid a unit with any significant degree of water damage.
$700 sounds high to me for the propane tank work, but I'm not really up on the going rate for such things. Once inspected, you certainly could repaint it yourself; that's just standard metal refinishing.
You will need a lot of solar power and battery storage (and a good sized inverter) to reasonably run an air conditioner off of solar power. RV rooftop air conditioners are none too energy efficient, either; a good modern mini-split system could cut the energy use by a third for equal cooling, and even a decent household window air conditioner would also be a significant improvement.
Frankly, if you don't need to move around, I think there are other things that would be a more useful starting point for your project. A mobile home or a largeish kit shed come to mind as possibilities. If you need to be mobile or somewhat mobile, then obviously those wouldn't work so well.
