First trip out in the Motorhome - questions!

1. Fault with heater - When we left the motorhome for a few hours, we left the heating on electric low setting and upon return the red LED light was flashing 4 times before pausing - I looked at the troubleshooting guide and it said something like 'check combustion inlet flue' or something...any ideas? I reset the main isolation switch and then the heating came back on. The same fault occurred the following day but when i accidentally turned the master switch off on the control panel. When i put the master switch back on the heating came back on...

2. Due to the heating being on for 2/3 days on low, we had a problem with condensation at the front of the motorhome....solutions?

3. Upon draining the water system before leaving site, we opened the blue tap and grey tap on the outside of the Moterhome, the blue tap started draining but hardly anything came out the grey waste tank...(we only used approx 2 sinks full of water for washing up as used site facilites). Does the grey waste have to be at a certain level before it starts to drain? This doesn't ring right as we were told to completely drain the system to stop any frost damage etc when in storage..

any tips much appreciated...