If you have a flat tank and it`s say 14" high you lose no room hardly at all lol how can you say you lose room,all you`ve lost is that low level space.With an upright you simply just lose that space you have no room to store on top with a sheet of ply etc.
NWH that would be fine if what I wanted to put on top would be ok, it will not as it weighes to much, as I said I have both, and flats take to much floor space and putting them sides ways in van would worry me as I have it length ways and it rocks front to back half full when stopping, i'm looking for a solution to space and safety