Depending on how thick the flooring is, you might have to upsize the anchor bolts, so they can withstand the bending forces as well. Or, you can install a sleeve to take up the flooring thickness between the van's floor, and the bottom of the seat base.
FWIW - DOT doesn't "approve" anything. The FMVSS sets standards, and the OEMs and aftermarket converters establish compliance with those standards. For passenger seats, the requirement is 5,000 pounds per anchor for lap belts, or 3,000 pounds per shoulder belt anchor.
Legally, you have no exposure unless you later tried to sell it having compliant seats, when you didn't test them. Of course, you want to do it right for you own family's safety....
Note that the RV industry is not required to meet any rear seating safety standard. So most aftermarket seating options are not any safer than a park bench.
I'd suggest some nice take out OEM seats, using the OEM base plates, and mount them with OEM or larger anchor bolts. You'll need to add a backer plates as well, if you aren't able to use the factory backer plate which will only exist where the OEM rear seats would have been.
Transit seats might be a good option, because they have integrated shoulder belts, and most positions have the LATCH system. Beware they come in varying widths, so you'll have to figure out which width will fit your layout and carseats the best.