Hi Bavnav, I have a B30 and also had the rogue LED flashing until recently. First of all, the holding tank, sensor and inspection hatch are all accessed from the hatch to the right of the vanity mirror in the heads. If you reach behind the tank with a torch you can see the level quite easily because it is transparent The tank is emptied by gravity by opening the large seacock under the sink. If the tank us full or empty you won't see the level so close the seacocks and pump the heads a few dozen times. You will either see the level rising or if the tank is already full, water will squirt out of the vent just aft of the heads on the outside of the hull near the toerail.
The sensor is probably faulty. It is difficult to remove because of lack of clearance but I have had mine out for a thorough clean but it still gives a false reading. Eventually it snapped where it has corroded trying to put it back. I have decided it is pretty useless anyway so have just disconnected the wires to stop the LED flashing. If it is ever likely to be full I will know, and I can always check the level with a torch.
If you find that the tank outlet is blocked (unlucky) as I did once, you can't open the circular hatch on the tank because it is full so you either have to get it pumped out first or live dangerously and pressurise the tank to blow out the obstruction. This is what I did. I cobbled together a seal for the vent and used the Jabsco flush to provide the pressure. Take care because it only took a few strokes to get the whole thing creaking and popping. I was just beginning to worry about the tank splitting or pipes blowing off when whoosh - instant relief, as it were.
As someone mentioned, it doesn't have to be foreign objects causing the blockage, it can be scale. What happens is that if you leave the tank empty for a while, the scale on the sides dries out and falls to the bottom in flakes that cover the outlet. That's ones theory anyway.
By the way if you are going to do this in a marina, make sure it is mostly water in the tank.