Storing a Diesel Motorhome for 5 moths in Las Vegas

I would top it off and put a double dose of Bio-bore or some other brand of biocide in the tank and run it a few miles. While you'd probably be OK without it, it's cheap insurance against potentially many thousand of dollars in repairs.
It would also be good for the batteries if you could put a maintainer (not a trickle charger) on them during storage.