Hi JSMC
I see that they both have thermostats, but you need to check if they can be set to a couple of degrees above freezing.
You only need to keep your van from freezing - not turning it into a tropical climate zone.
Ours are set just above freezing and they cut in and out during the night and neither van has frozen this winter yet. They are 750w converted wall heaters, now fixed to a steel frame to make them portable.
Cameron has a Peugeot Partner and I have a Citroen Relay.
Cheers
Spruce