Fiat Lunar Champ A521 mains not charging

I have an A521 Lunar Champ - the leisure battery and the van battery are both being charged when driving the van - however when on a site yesterday I found that the leisure battery was nearly flat and the van battery half full - if I switched the lights on they would go dimmer until the 12 volt switch on the panel tripped - I have checked all fuses on charging unit and they seem to be OK - on our way home today the leisure battery has become fully charged - the charging unit is a CALIRA BC723 - anyone any idea what the problem is ???

Thank you