Old Zig Unit

Can someone help? I have a 1980s caravan, which has a problem with the zig unit. I plugged it into the mains switch it on and the battery condition light didn't come on. The charger light is on, fuse ok, but i all not getting any voltage at the battery, Everything else is working, ie mains sockets & fridge, & all the 12 volt stuff. Do i have a problem with my charger? Is it fixable or is it a new unit?

Any advice please.    Paul