Now that the fun’s out of the way…that actually sounds like an interesting idea.
Couple things to keep in mind…
Will it remain set up when moving from place to place?
If so, I’d go bare bottom (not stirring up sand with bumps and what not).
I’d also go acrylic as the bumps and jolts may accelerate potential seam failure/glass cracking (I could be making this up, I honestly don’t know).
@Joe Glass Cages may be able to chime in here)
I’d also make sure it has a eurobrace style lip for splashing purposes.
Also make sure your rock work is stable. Perhaps consider placing acrylic rods secured to the tank to zip tie your rock work to or feed your rock work onto to keep it from shifting.
Will you have a plan if the RV breaks down and needs to towed?
I’d also consider the smaller the tank the better in this case.
How do you plan to secure the tank within the RV it’s self so it’s “shelf/stand stable” per se?
Just some things to think about.