The only reason the 1 watt solar charger is keeping a charge is your battery shutoff switch. At 83 milliamps of charge in full sun it will just barely replace the current loss due to internal resistance in the battery thereby keeping the battery "topped off" as you stated.
Since the Propane detector uses 100 milliamps of power when the battery is left connected there will still be a net loss of power over time (especially since the propane detector runs 24/7 and the sun does not).
Dead battery will just be measured in weeks instead of days.
For a new installation a 10-15 watt panel would be a better choice along with a battery disconnect. That would keep the battery "ready for camping" longer.
NOTE: What these small panels will not do is charge your battery if it was discharged at all to start with.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW