These produce symptoms similar to a dead cell - the lights and heaters work reasonably well, but no amount of charging will get the engines turning over briskly. Check connections in the battery boxes, the battery switch / fuse box, and on the starters themselves. Immediately after trying to start an engine (ie within a few seconds) Feel all the connections with your hand to find any warm ones - bad connections can generate a lot of heat. In extreme cases, sparking can be seen / heard. Yours truly has learned the hard way that bad connections or one kind and another are a major cause of problems!
The biggest problem is caused when a cell goes 'dead' and will not hold any charge. This can be caused by the plates warping and shorting out, which tends to happen on the old black rubber cells, often accompanied by the top of the cell lifting away. Other failures include sedimentation at the bottom of the cell shorting the plates out and / or the active material on the plates being destroyed. Repeatedly running a set of batteries flat is a good way to kill one or more cells by over-discharging it; regular overcharging can loosen the active material, which then falls to the bottom of the cells and shorts them out.
A totally dead cell is usually obvious using a hydrometer or multimeter; what you should look for is one or more 'odd ones out' in a set. The dead cell will have a much lower voltage after charging than the others (or even a reverse voltage - beware with a digital meter, it is easy to miss this), a much lower hydrometer reading, and may get warm during charging. It will also use little electrolyte, and the other cells in the set will use more than usual. Cloudy (ie muddy-looking) electrolyte is another bad sign.
Having identified the failed cell(s), it / they should be replaced by one of a similar age and capacity. It is important that both the replacement cell(s) and the remaining good cells are fully charged before use - never put a flat cell into a charged set without an equalising charge, or it will never charge properly and soon fail.
This usually indicates a charging circuit fault or a failing cell - see 'dead cells' above, and also the fault-finding notes for alternators and generators. During charging from a battery charger, check the voltage of each cell individually with a digital multimeter - it must be at least 2.3volts, or the battery will not be fully charged. The charging current will drop to only a few amps once charging is complete. There may also be a continuous drain of some kind on the batteries - see below.
From a fully-charged state, there should be enough power left in a set to start engines after at least 3 months. If not, a slow discharge is usually indicated, often through earth faults. You do turn the battery isolating switch off at the end of the day, don't you? Not doing so can leave a small electrical load, even with the keyswitch turned off, depending on how the vehicle is wired.
With everything switched off and any charging lead unplugged, remove one of the battery connecting straps and connect an ammeter in its place. You will be pretty much guaranteed to get some reading, with most vehicles' wiring being many years old - perhaps 10 milliamps (that's 0.01amps). Such a reading isn't a disaster; it will take many months to drain a battery at that rate, although it will shorten battery life a little. If it's much more than that, you need to start looking for the problem - 100 milliamps and your batteries won't start an engine after a month or two. If it's an amp or more you've either got problems, or somebody has left the cab light on or the charging lead plugged in!
To narrow down the problem, disconnect all jumpers from other vehicles - under some circumstances currents can circulate through the jumper connections. Note the ammeter reading, and then take a fuse at a time out of the distribution fuse box, checking the ammeter reading each time. This will obviously tell you which circuits you need to start investigating in detail to locate the problem(s). Heater glow plugs seem to be a favourite for producing earth faults.
Voltage gives some idea as to how charged a cell is, although a hydrometer reading is more reliable if previous readings are available for comparison. As a rough guide, with the lights switched on and the charger unplugged for a few minutes:
Below 1.83V / cell: very flat, recharge as soon as possible
1.83V / cell or 22.0V total: flat
1.92V / cell or 23.0V total: nearly flat
2.0V / cell or 24.0V total: ought to start an engine but requires recharging
2.08V / cell or 25.0V total: OK
2.17V / cell or 26.0V total: good
Typical hydrometer. Dependent on the state of charge of the cell, the specific gravity (density) of the electrolyte changes, and the float sits higher or lower in it (the grey bit is the electrolyte). Make sure that the float isn't hitting either end of its travel and giving false readings. Hold the hydrometer vertically and read from the lowest point of the meniscus (that's the curved surface you get on the top of a liquid) as shown.
Again these will vary from vehicle to vehicle, depending on battery age and so on. As a guide:
1.150 or below: flat or dead
1.200: nearly flat
1.250 or above: good. Most batteries don't get much above 1.250.
When trying to trace problems with poor starting, bear in mind that starter motors can get 'tired' for various reasons; for example, oil can contaminate the brushgear, and the motor will still work, but not very well. This can easily be misdiagnosed as flat batteries or poor connections. Such problems are often indicated when a vehicle has one engine which is 'easier' to start than the other.