Can I use a van battery with an invertor to power a food warmer?
You calculate the energy required by multiplying power by time.

Your food warmer requires 2000 W for 2 h so energy required = 2000 x 2 = 4000 Wh = 4 kWh which would cost you in the region of €0.40 to €0.80 per use if it were mains powered.

Your battery - assuming it is a 12 V car battery - is rated at 70 Ah and since power is volts by amps (P = VI) we can calculate the energy in the battery = 12 x 70 = 840 VAh = 840 Wh.

You are short by a factor of five in perfect conditions. Since you don't want to run the battery flat (as that shortens the life) you would probably need about eight batteries. These could be wired as four parallel pairs (24 V) or two sets of quads (48 V) for a higher input voltage inverter as these are more efficient.

See if you can use gas!



From the comments:


  
  The food warmer in the 2 hours operation I think will not be on all the time as will stop all the time it reach the right temperature. 
  


Correct. So total energy consumption will be 2000 W x run hours. (The answer will be in Wh).


The van battery (12 V) will be recharged by the alternator while the engine run .


Current required from the battery will be given by \$ I = \frac {P}{V \times e} \$ where e is the efficiency. If you were able to get 80% efficiency (0.8) then \$ I = \frac {2000}{12 \times .8} = 208 \text A\$. It is unlikely that your alternator could provide anything like this much current over a long period and at low engine revs.