The symptoms as you correctly believe are classic of the heating circuit being "air-locked" and needing to be bled, or force purged.
I can't help you in directing you to where all the bleeder points are with this van; my Hymer has 5, each on a tee rubber coupling tube joining the rad's to the pipework. These were very hidden and required me tracing the tubes all round the van and dismantling bits of furniture. A torch and hand held mirror helped in the blind areas.