From the manual cover, this is a diesel parking heater (a common Chinese-made "Webasto/Eberspächer-style" unit), not a heat pump. That matters, because the symptoms you describe point to a few specific causes.
Important safety note first: If it's venting exhaust into the van or you smell fumes inside, shut it off and ventilate. Diesel heaters should have a sealed exhaust pipe routed outside the vehicle. Make sure you have a working CO alarm in the van.
1. Combustion isn't happening properly (most common) Fuel is being pumped, but it isn't burning well, so little heat is produced. Signs: white smoke from the exhaust, a fuel smell, or the unit cycling through start-up repeatedly.
- Carbon-clogged burner or glow plug: Remove and clean the glow plug and burner chamber. Replace the glow plug if it's old or cracked.
- Too much fuel / over-fueling: Check the fuel pump rate and make sure fuel lines have no air bubbles.