First, pump and rinse the tank--which should be done several times a year anyway to eliminate any sludge...add some water, then use the muriatic acid solution. All that bubbling will certainly let you know how open the tank vent is!

If it hasn't already occurred to you that the head intake line also needs cleaning out, it does. Easiest way to clean out the whole system: disconnect the head intake line from the thru-hull (might be a good idea to close the seacock first) and stick it into a bucket full of the acid solution...flush it through to the tank.