What you need to do is use an ammeter in series with the battery, then start disconnecting things until you find the circuit that is drawing the current.
The problem is that most ammeters that are incorporated into multimeters are only good for a fraction of an amp. There are many newer digital multimeters with a 20Amp capability. It very well could be that your current drain is less than an amp, since it takes a few days to drain the battery.
First you need to ensure that the battery is being drained, rather than not being charged. Use a DC voltmeter across the battery to measure the voltage. Measure with a charged or mostly charged battery and the engine off, the voltage should be around 12. Then measure with the engine running, the voltage should then be about 13.5 give or take. If the voltage stays the same after starting the engine, the battery is not being recharged.
You can make an ammeter with a heavy .1 ohm resistor and a voltmeter and a simple calculation.
Best of luck,
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