There's no way that a voltmeter can truly test the health of the batteries, since a perfectly crappy battery can read 12.88 volts at rest after being fully charged, but drop to below 12 volts when a load is applied. Only way to test them to see if they can hold a charge is to use a battery load tester. Any repair shop will have them, most places that sell batteries will have them to test your batteries. Walmart, Costo, Pep Boys, Autozone can all test them for you. You can buy load testers. The ones under $100 will generally tell you good weak or bad, and the ones in the $200-$400 range will tell you more information about the actual capacity of the batteries.
A load tester removes amps from a battery much like starting an engine would. Some battery companies label their battery with the amp load for testing. This number is usually 1/2 of the CCA rating. So, a 950 CCA battery would load test at 400-500 amps for 15 seconds. A load test can only be performed if the battery is near or at full charge.
You can still use a voltmeter to test them, however. Fully charge the batteries and then do one of two things to remove the surface charge. Either, remove all connections to the batteries for at least 12 hours, 24 is preferred, or the battery must be discharged for several minutes to remove the surface charge (headlamps on high beam for 3 or 4 minutes will do it). Then remove all connections to the batteries, and of course disconnect them from each other. Then test the voltage.
100% charge - 12.7v
75% charge - 12.4v
50% charge - 12.2v
25% charge - 12.0v
DOA charge - 11.9v
I don't know what kind of batteries you have, but being 950 CCA batteries indicate cranking batteries, and being in a truck it suggests Group 31 truck batteries. Group 31 truck batteries of 950 CCA generally have about 110 Ah and 195 minutes RC (Reserve Capacity minutes at a 25 amp draw). Four of them gives you 440 Ah and 780 minutes RC.
780 minutes is 13 hours. But the RC is the number of minutes a fully charged battery at 80 ° F will discharge 25 amps until the battery drops below 10.5 volts, so you won't get anywhere near 13 hours out of them at a 25 amp draw. The inverter (and anything plugged into it) will start screaming or quit working at closer to 11.8 volts.
Truck batteries and marine batteries are the same thing, BTW. The only differences are the types of terminals they have, and the marketing and packaging. But internally they are the same.
There are cranking batteries that should be used for cranking. These generally have CCA as the most prominent specification on the label. These have thin lead plates that are designed to provide a large amount of amps (for cranking) for a few seconds. They are not designed for even low amp draws (1 or 2 amps) for extended periods.
There are deep cycle batteries that should be used for deeper and higher amp draws, and will have the amp hour capacity as the most prominent specification on the label. These have thick lead plates that are designed to provide plenty of amps for extended periods.
Then you have the hybrid batteries that are somewhere in between cranking and deep cycle. These are called truck and marine batteries. Some of them even have "deep cycle" on the labels. They are simply deepER cycle than cranking batteries are. They are designed for the same type of low amp draw that a trolling motor or running lights on a boat requires. This means 5 amps or less for extended periods.
A laptop and lights, or a TV and lights will usually draw at least that much via an inverter. The inverter may be drawing 1 amp all on its own, the light another amp, and the laptop or TV another 3-5. It adds up. Throw in a fridge and you're in serious deep cycle territory. All that will certainly work on four hybrid batteries, but it's a tremendous strain on the batteries, the batteries should be closely monitored with a Battery Monitor to ensure you don't discharge them too deeply and that you full recharge them when you do charge them back up.
One of the worst things you can do to a battery is run it down to the point where things stop working or the inverter starts screaming, and then crank the engine and let it run for 15 or 30 minutes thinking that's enough to recharge the battery. More likely the engine needs to be running 8-12 hours to recharge the batteries. This is also a very common scenario. Discharging too deeply and then not fully recharging the batteries is the reason the saying "Batteries don't die, their owners kill them," is so true.
On a boat you will have one battery (or battery bank) that does nothing but crank the engine and run the boat's electrical system of marker lights. You will have another totally isolated and separate "house bank" of batteries to run everything else. If you run the house bank down, the starting battery bank can sill start the engine. It could mean your life. The concept in an OTR truck or van is no different, where you want a cranking battery that does nothing but service the truck and a house bank that services you. If you combine the cranking and house bank into the same batteries, it might not mean your life in a truck, but it'll certainly mean annoying jump starts and frequent battery replacements.
