If your valve clearances keep tightening up then you are suffering from valve recession.Propane is hard on both the valves and valve seats. If the mixture is slightly rich this will make the situation worse as combustion temperatures will increase. On propane powered engines I used to run one of...
What size power cables are you running from your battery bank to your inverter? Have you done a voltage drop test between the battery and inverter. Low voltage to the inverter can cause the problem you are having. Just a thought.
By some 50 x 50 aluminum angle and cut off 100mm lengths. Tech screw the angle to the edge of the panel and then to the roof. Sorry if the metric measurements confuse you but here in N.Z imperial isn’t a thing.