Technical Batterry/Electrical issue? (Doblo stop

[TX22]

Hello everyone! First time here

Since maybe a year I have a problem with my Doblo 1.9...

1. First the radio turned off and random lights appeared (Airbag, belt, abs...). I went to a local mechanic and they replaced my battery and glow plugs (all broken). The ECU showed a "connection problem.
2. After 3-4 months same problem, random lights appeared (mainly ABS light), sometimes engine stopped running while driving, other times immobilizer activated. I went to another mechanic, same ECU message "conection problem", glowplugs broken again and they said that it could be a problem with the ground wire (when they tried to start it, it gave a voltage of 6v in the battery, but after a few minutes it gave 12.8v again and it started without problem). They tested it for a week and returned it to me.
3. After 3-4 months, the SAME PROBLEM AGAIN. This time they checked all the negatives, including the fuse box (?). They tested it again for a week. At this time I installed an aux battery inside, connected to the main battery with a battery combiner (for a small fridge and some led lights), I checked the voltages and they were normal in both batteries, 12.7/12.9v stopped and about 14.5 driving.

A week ago the ABS light appeared again, and I see that the voltages in the main battery are 12.5v stopped / 14.2v driving and 12.7v stopped / 13.6 driving in the aux battery, dont know if it is normal with 35-40ºC outside. I'm going to take it back to the mechanic next week, but any idea what could happen? I've seen several similar threads on the forum but I don't see any clear answer...

Thanks!