Finally fixed it. Tried to order the sterling 30A, but it was never delivered.
Meanwhile I removed the Univolt and bought and installed the Blue Sea 12V
fuse panel. I cut out the part that measures the amperage, the shunt. Then just monitored use for a few days. It never actually went over 10A. Not even that: most of the time round 5A. And the occasional peaks (waterpomp, heater fan, iPhone charging thru radio/CD) can easily be dealt with the capacity of the revived battery. And we're never using 10A the whole day, at night we sleep, at least most of the times.
So why buy 30A? And why a bigger battery? I just decided to stick to my 55Ah battery and bought a small C-Tek 7A smart charger, only 0.8kg, and works like a charm. Has 8 different stages for reviving, charging, and 12V
supply. Love it.
Found out my other battery is for brakes only, and is charged by the car while driving. So I'm pretty much done.
Only thing left now is to replace to 110V fuse panel with a modern 220V. Bought one, just needs to install giving really lousy manual.
One thing I really discovered is that almost anything I need for my Airstream in Europe, can be bought at yacht stores!