A split charge relay can be useful
The amp per hour charge rate is dependent on a number of factors for example the length of time the engine/alternator are running and how quickly they run.
Most alternators are designed to trickle charge a battery so the longer and faster they run the higher the charge rate. (Up to the maximum capacity of the alternator and battery) There is also a restriction based on the maximum capacity of the alternator and battery in any system plus any drop off through the wiring due to resistance.
The ampage per hour rate will drop off as the battery nears full charge, the batteries capacity to take amps drops as the voltage capacity nears its maximum. The only way of physically pushing higher current into the battery would be to increase the Voltage capacity and increase the maximum generating capacity of the Alternator.
Ohms Law
http://en.wikipedia.org/wiki/Ohm%27s_law1 Despite the split charger the battery is still not being re-charged fully. Take it out of the van or leave in situ and re-charge each night with a battery charger.
If you take 4-5 amps for 5-6 hours in a working day but spend only 2-3 hours driving (and charging via the split relay) you will simply not put back in, what you have taken out.
2 In the cold weather I think the water is actually harder to pump. The batteries will run flat quicker as the pumps are taking more current.
If the battery is fine then the system looks like it is causing too higher volt drop because it is taking too much current. Reasons for this can be: pump is old/worn, a bad fitting restricting flow or some other flow restriction (dirt, too small jets, too small hose etc). These are the next things to look at.