It looks as though you have a few odd days doing 30 years of maintenance and cleaning to catch up on a few items and maybe even some replacements.
That red 'thing' in the propane line is an 'on' indicator. Our long gone '74 Kountry Aire 5er and '78 Rolls TT had one like that - and both got replaced after quitting.
You really do appear to need the usual fire/CO/LP detectors as well
The HW heater is a different beast then anything I've ever seen and the valving -- ?????
Marathon tires if made in the US, Canada and Mexico have been pretty good. Those made in China have been labeled with the Tow Max as China Bombs. If they are more then 5-6 years old, they are suspect regardless of sidewall or tread cracks. Then you add in sitting dormant for an unknown period of time. That spare is questionable and might even be an original tire
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Wheel covers - why not see if craigslist or a junkyard might just have a set of 4 identical and replace them all. Best and mostly for appearance would be a nice set of aluminum wheels which would really set that unit off.
I'm not sure if that Coleman a/c can be serviced if it has (or will) quit.
It's seems like a beautiful TT and a lot better built then most 2015 offerings. Why not start a thread in the Vintage section and describe your efforts to bring this nice unit back to the world