My SWAG is it would be OK, at least as long as the van isn't running. Not sure about having it turned on without a house battery connection and feeding altenator current to it. That might be similar to connecting solar panels without a battery, first.
If you want to be extra careful, move the starting battery connection, on the charger, to the house battery position. They're real close together and I would think you'd have enough slack in them to do so. It's only two screws and a little time. I would do the same, if I was to connect solar panels to it, without a house battery. For testing/setting the solar settings.
The other thing you could do, is connect a old LA battery as the house battery. If you have an extra hanging around not being used. If I did this, I'd follow the manual when picking the Battery AH rating.
One other thing, download a PDF of the manual and print it out on 8.5" x 10". It makes it much easier to read and follow along. I made up a binder with it inside. It also allows you to put the setting flow chart out as one big one. I wish I could get a PDF of the flow chart as one page and have it printed on a 11" x 17" page as a wall poster for setting it up. I think I'm going to email Kisae again and ask them to do just that.
Anyone wanting a PDF of the manual or spec sheet, you can download it here. It's under related documents.
https://www.donrowe.com/KISAE-DMT1250-Abso-50A-DC-DC-Battery-Charger-p/dmt1250.htm