Not for very long, though - and if you are running it off your starter battery you probably won't get going the next day !
800w at 12v is nearly 67 amps drawn from your battery & inverters are not 100% efficient. Most starter batteries on vans are 110-130Ah & you are not supposed to run them down to less than 50% capacity for reasonable life, so you will get less than an hours heat at best.
A better (but pricier to install) option for reliable overnight warmth is a Webasto or Eberspacher diesel space heater - frugal on diesel and battery usage. Used ones come up on Ebay, but could cost more to overhaul / fix than new kit.