Either the 75 amp charger is defective, or it is being "pulled down" by one or more defective batteries, which may not be seen as a voltage drop when running on the alternator due to the much higher capacity of it compared to the charger.
I would park with engine off and inverter off and check the specific gravity of each cell with no loads. A bad cell will generally show up if you have one.
If nothing shows up purchase a 10 amp "fully automatic" battery charger with tapered charge reduction ($30. to $40.) and put the batteries on charge. Having been fully charged from the alternator, they should not take more than a few amps (with no loads on). If after several hours the batteries are taking little or no current, the problem is a defective 75 amp charger, which will eventually ruin the batteries -- if it hasn't already.
If the 10 amp charger pegs the needle and it stays there for several hours (with all loads disconnected), then you have one or more defective 8D OR a current leakage from a connector. Make sure connector and battery case are all very clean to eliminate leakage current to chassis ground.
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Hopefully this helps.
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