I used ACX plywood, 3/4". I considered marine grade plywood, but both use the same exterior-grade glue, the biggest difference is that MGP has fewer voids than does ACX. That is important when building a boat, but less important for the floor of an RV. And ACX is maybe 1/3 the cost of MGP, so that was certainly a factor.
I picked it up at Austin Fine Lumber, it's not available at any of the box stores as far as I know.
You asked where the faucet will go? Behind the sink toward the curbside wall. I will post pictures when we get it installed. To cold right now and we kinda wore ourselves out getting ready fro the fall rally in Oct this year. So much to do ... so little time.
Thank U for the post.