You should start by checking all connections, including ground. Remove and clean each connection, one at a time. Check that the electrolyte levels are all above the plates visible in under each cap. You should also take voltage readings of each individual battery as you disconnect and clean the connections. A good 6v battery should read about 6.25-6.37v. If no water needed to be added, you can at the same time use a hydrometer to do a specific gravity test.
You mentioned you have 2 solar panels, but how many watts are they? In addition to solar charging, you should have a converter/charger that works on 120v power, either from the shore cord or the generator. Test by taking voltage readings at the batteries when shore cord is plugged in. A converter/charger should have an output of at least 13.4v when charging the batteries. Disconnect solar charging when testing the converter/charger. If your RV has a battery isolator that connects the house and chassis battery systems when a charging voltage is detected, the alternator of the engine will also then charge all batteries when the engine is running. Verify if this is happening by taking voltage readings when the engine is running.
A reading of 3/4 is sort of meaningless unless you know what voltage 1 equals. Use a voltmeter that is calibrated in volts. 4 six volt batteries will have much more storage capacity than two 12v batteries.
House batteries should be deep cycle batteries that give out relatively small amperage over a longer time period. Starting batteries are good for chassis batteries since they are designed to give the necessary short-term high amperage to start the engine.
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