If it's a new van I would give the tanks a couple of flushes while you have it wrapped up, just to get any residual out, using drains and the taps, otherwise I think it doesn't really matter if the tanks are full or empty so long as you've had good water in them. Depending on your water, it may be advisable to drain your HWS, (always make sure you turn the power off to the HWS when you finish using it, saves problems later on), if its good water then leave it alone. Mine usually has rain water in it and I just leave it as it is from my house water tanks and it sits there for 6 mths or more and is good, van tanks gets circulated more often actually.
My rig gets used about 9 mths a year so my anode last's for 2 yrs or more so if you have it parked up for months at a time, check your anode (in HWS, do a search) at least every 6 mths as lack of use and dodgy town water will eat them inshort time.