There have been several threads regarding tank valves. Don't know about the Vantage, but our enclosed underbelly is covered with a corrugated plastic about 1/4" thick. If you can locate where your tank valve is by looking for the actuator rod, you can remove a few screws along the edge of the coroplast and determine where to make a three sided cut to make an 'access' door.
A forum search for black tank valves should get you the threads.
Jack & Marty
2018 Laredo 298 SRL
2011 F-250 SB Crew Cab 4x4 6.7L