Questions on rechargeable battery pack configuration and calculations
For batteries in series, voltage adds, but ampere-hours are unchanged.

For batteries in parallel, Ah adds, but voltage is unchanged.

So, your 20, 1.2 Volt, 2450 mAh cells in series will give you 24 Volts at 2450 mAh.  Five such packs in parallel will then give you 24 Volts at 12,250 mAh.

You would be much better off using two large 12 volt deep-cycle (also called "marine" or "RV") lead-acid batteries.  A Group 27 (somewhat longer than a traditional car battery) lead-acid battery is about 100 Ah.

Even then, you may have to plan on charging the batteries from your van's alternator while driving.