Brandon, welcome to the forum. I am a little confused with your question, but will try to help.
The title says "Issues with my LP Gas detector". Typically, the LPG detector is low, just above the floor, since the fumes are heavier than air and will tend to collect low.
I do not understand what is meant by your "unit" that has the red light next to it. Is this the detector, down by the floor, or is it the switch panel for a water heater? If it is for the water heater, the red light means the heater has not started and requires a reset/restart. Simply turn off wait a few seconds and then back on. If the red light is for the detector, could it be low battery power?
Lastly is the propane tank. If the lines were completely empty and the tank was refilled with the outlet valve closed (proper procedure by the way), upon reopening the valve too fast, can engage a safety device that will prevent gas flow. It's a valve that closes automatically if there is too much flow, indicating something's wrong and closes off the flow. Resetting the safety valve is as simple as closing the tank outlet valve and waiting 10 to 15 minutes. During that time be sure all appliance valves are closed. After the wait time open the tank valve very slowly so as to not slam the pressure to the system. Finish by opening the tank valve all the way and try starting you appliances.