Twop thoughts on the batteries and propane may help. On the propane, when you have the tank filled, ask them to look at the date and I think that is a factor on mobile tanks, so perhaps also on permanent mount? The guys who fill tanks may also have folks in mind who do things like change the valves and hoses, so we often go to the places which are less "special" shops for RV as it seems to work better if the place just does propane!
It seems like different folks have different thoughts on working on Rv and someseem to think it very special while others have done propane on lots of differewnt types and see nothing different to bother them. Some calling around after searching for propane or liquified gas may turn up some good places.
The sad story on batteries is often that they are not old, just abused to death!
Without doing something to make sure All the batteries, both start and coach are kept charged, there are slow drains on both sections which make the batteries run down while the RV is setting, either on a sales lot or stored.
When a batery is run too low and left for too long it does lots of damage, which make mean a three year battery goes bad in hal that time.
So the big point is to keep those small drains from killing your battery! Radio presets can kill the start battery and safety items like propane and CO detectors are NOT cut off by the coach battery disconnect, so stay alert and make some system to either totally disconnect them or to keep a small charge going to them.
I might guess that most used Rv that set on lots will be sold with half dead batteries as they make no effort to keep them charged!