Battery has to be bad or something along those lines. We have not had problems with the Sienna battery, but I’ve had to replace "new" batteries in new vehicles before. Manufacturers use the smallest and lightest battery possible to start the vehicle, and if the battery is damaged somehow in transit (froze, dropped, defective, etc) it may struggle to produce enough cold crank amps to start the vehicle. I had a Subaru Outback battery that the dealer insisted was fine, would recharge it, and the vehicle wouldn't start the next day. This went on for a while. I eventually bought a new battery from Autozone and never had another issue. I suggest, if your dealer refuses to help as you say, buying a new fresh battery to see if that solves the problem. If the van is still failing to start, then you know that you have some type of parasitic draw on the battery or other electrical issue which unfortunately the dealer will have to solve.
Two other general things I just thought of that can cause a small draw on the battery: the key being stored near the van, or leaving a door or the rear hatch open for a long period of time (even if interior lights have turned off).
Good luck trying to solve this issue. It's very frustrating when any car, but especially a new car, has trouble starting.