The Mercedes Benz Sprinter is a popular trade van and motorhome. It comes with a factory immobiliser and keyless entry, however, it does lack an alarm.
My preferred alarm is the Cobra AK4615 CAN-BUS upgrade alarm which integrates with the factory remote.
The Volkswagon LT/Crafter is a re-branded Sprinter and the Cobra works fine in that too.
Mercedes Benz Sprinter Alarm
Cobra Alarm Installation Details
The Ultrasonic Sensors can be placed either side of the sun visors, or in the centre roof pod that houses the internal lamp (see photo below), the later location works well for camper vans where the vision to the back of the van is often blocked by side cupboards. A PIR sensor can also be added to cover the back of the cabin, and reed switches can also be used if additional hatches require protection.
For those wishing to lock the van without arming the alarm (motorhomes) the doors can still be locked using the lock switch next to the hazard warning button. This way the doors can easily be locked without arming the alarm when sleeping inside the vehicle.