Welcome to the site!
A Tracker is for getting the van back should some rogue steal it, and will likely reduce your insurance premium too, but an alarm protects it from the more likely scenario of break ins and petty theft.
Several companies specialise in motorhome security and would be well placed to advise - I have dealt with VanBitz in the past and they really know their stuff, offering tracking and alarm systems. Maybe hit the phones and ask one of the specialist security firms what they'd recommend based on your van and what you want to achieve!