The best check is to put a charger on it and see how long it takes to fully charge the battery.
One of the problems is that a starter battery will appear to be fine in the morning but flat in the afternoon.
A starter battery isn't designed to slow power equipment. Its job is to start the engine. Once that's running the alternator powers all the accessories and tops up the battery.
Each time we start a 2 liter diesel it is estimated the current draw is about 5.4amps if you spread that over an hour rather than just a few seconds. That's a little more than a Shurflo pump draws in an hour of washing the windows.
So you can't expect the battery to last as long as it would if you were just using it to start the van.
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