A tenth of a volt drop every couple of hours is very roughly 10% of your battery capacity. Your battery is probably a Group 27 so 10% is probably 7 amp hours. That is a huge "parasitic load", so something is going on.
Beg, borrow or buy a DC clamp on ammeter. Klein Tools is a decent brand and they are $90 on Amazon. See-
https://www.amazon.com/Klein-Tools-C...96&sr=8-3&th=1 Every DIY should have one.
Then measure the current from each wire attached to the positive terminal of the battery. One should be high. Then see where that wire goes and trace the current down. I'll bet it is a relay like the starter solenoid on the generator that has gotten stuck and draws current continuously.
Until you find it, disconnect the negative cable to the battery which will shut down all circuits.
David