Something doesn't sound right. 20 minutes work is nothing. The pump has probably drawn 1.5 amps in 20 minutes.
When I put my son's Partner van into service I fitted a change over switch so he could use the van battery if needed. His Jack Russell dog knocked the switch from the leisure battery to the van battery one day.
Within 4 days his van battery was flat and he had to jump start his van. We do minimal mileage so we put it down to that.
Then his van battery failed and was replaced. Out of interested we tried to use his van battery, and again, his battery was flat after 4 days. Yet another windie who used to visit this forum successfully ran his system from a Berlingo van's battery.
We later brought son in law into the business and fitted a system in a Transit Connect. He wanted to use the van battery rather than buy a leisure battery. Although he does more mileage than we do, his van battery was flat within a week.
Webasto make diesel heaters which are primarily block heaters. They are designed to heat the engine water in freezing weather and defrost the car at the same time. This usually takes about 30 minutes. These Webastos will use about 12 amps to start (15 to 20 secs) and then use about 2.5 amps after that. The internal car blower motor will also use battery power.
Webasto advise that the only way to successfully run their system is that the car must run for as long as the Webasto has worked. So a 30 minute start would need a 30 minute journey to replace the charge back into the battery.
We only do a 5 to 10 minute journey to and from work each day, so the alternator will never replace the charge we have taken out during a day's work.
I would agree with Spruce. You also need to take into account that a van battery and leisure battery are designed to discharge/charge in a different way. Your Van battery is designed to provide a large amount of power over a very short time to start the engine. Once the engine is running the alternator will recharge the vehicle battery but it takes time to replace the large amount of current used to start the engine. The battery is designed for a high short discharge and slow recharge. It is not designed to be under load for long periods. The Lead plates and cells are thinner to suit the purpose
It is well worth mentioning the difference between the Leisure battery and a Vehicle battery.
A car's battery is designed to provide a very large amount of current for a short period of time. This surge of current is needed to turn the engine over during starting. Once the engine starts, the alternator provides all the power that the car needs and slowly recharges the battery. So a car battery may go through its entire life without ever being drained more than 20 percent of its total capacity. Used in this way, a car battery can last a number of years. To achieve a large amount of current, a car battery uses thin plates in order to increase its surface area.
A deep cycle battery is designed to provide a steady amount of current over a long period of time. A deep cycle battery can provide a surge when needed, but nothing like the surge that a car battery can produce. A deep cycle battery is also designed to be deeply discharged over and over again (something that would ruin a car battery very quickly). To accomplish this, a deep cycle battery uses thicker plates.
A car battery typically has two ratings:
CCA (Cold Cranking Amps) - The number of amps that the battery can produce at (0 degrees C) for 30 seconds.
RC (Reserve Capacity) - The number of minutes that the battery can deliver 25 amps whilst keeping its voltage above 10.5 volts
Typically, a deep cycle battery will have two or three times the RC of a car battery, but will deliver one-half or three-quarters the CCAs. In addition, a deep cycle battery can withstand several hundred total discharge/recharge cycles, while a car battery is not designed to be totally discharged.