You might get away with it but I think you are asking for trouble unless you visually inspect and maybe reinforce. I travel with just enough water for an occasional roadside flush/hand wash.
I understand a lot of boondock campers fill up as near as possible to their destination to minimize stresses.
There are various ways to cut an access underneath but you need to be careful not to cut something important above the plastic. You can Gorilla Tape them back in place or get fancy with some kind of hinge and positive lock for future inspections. I've never done it so I can't give any specific advice.
2015 Crossroads Cruiser Aire CAF27RL, 2018 F250 diesel crewcab SRW 4WD short bed, Trailer Saver hitch with Reese adapter on Ford OEM underbed 5th wheel connector.