Are you sure it's hooked to the gray water tank? That makes little sense to me. If indeed it is, then it may be a housing or storage tank for some sort of a filter system (quite possibly a reverse osmosis system with the gray water drain) and it's hard to suggest what to use for repair or replacement without a little more information.
If it's next to the water pump, it may be a pressure accumulator tank. A replacement would have to likewise be a pressure accumulator tank, but it could be a different style or construction. There are some for RV use that are plastic. (If that's what it is, it could also be bypassed safely, though the water flow is not as constant and the pump cycles more frequently without it.)
If perchance it's the water heater tank, it would not have a gray water connection, and it should not be replaced with a plastic tank. I don't think I've seen an RV water heater tank that looks like that, and in particular that is uninsulated, but there are many things I haven't seen.