hi, sounds like whats been used is under seal / bitchumin, what you can use on cars.
I would suggest, check over the whole area, any loose, flakey material, carefully scrap away with scrapper, and reapply with underseal, bitchumin, you can buy a gallon for about 15 to 20 pounds, and will last many floor coatings, to keep in tip top condition.
Another coating which is as good, is hamerite paint, or spray, ..
This paints as mentioned above are ok for metal floors, but if it was a old van and has wooden floor, clear yacht varinsh, by useing this you can see any deteration on the wood and can be repaired, but if coated in the above paints you wont be able to see clearly.
Hope this has helped. john