Bulkheads are walls, I think you are talking about the deck?
I would not use the spray-n-seal on the bottom of the doghouse but it may work well of the underside of the floors on either side of and behind the engine if you can get it cheaper than the Dynomat. If you can get to it the area above the transmission may be a great place for the spray.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad