#2: i was actually looking at propane detectors yesterday - they seem to run around 75mA (get a DC one if you can - I was looking at a bunch & this one looks nice - https://www.amazon.com/Propane-Alarm-Briidea-Detector-Black/dp/B098D2P56Z), youll need a constant power source - short term (like, a few weeks) one of the larger powerbanks would probably work if your propane detector uses USB for power - otherwise it's the whole solar system setup or a dc to dc charger (or generator) & a battery.

no idea on the other questions - my build will have an undermount horizontal tank - pricy but I have massive undercarriage space.