Fill the diesel tank to near the brim - so leaving no oxygen in the tank which encourages the slime-growth which can occur if left standing.
Can't remember the scientific name - but it looks (and feels) like snot - it chokes filters and pumps - horrible stuff to have to clean out of the tank.
Motor-homes / campers suffer bad from it.
leave windows slightly open to prevent condensation but only say 13mm gap
Come one, we are only talking about three months here....I used to regularly leave a car in airport parking for significantly longer.
Park it and leave it. Disconnect the battery if you must.
but only say 13mm gap .HTSH