Todd makes very high quality tanks in an assortment of sizes and shapes. I've used them for years on boats. They make a 20 gallon which is 30 by 16 by 10 inches which might fit under your Sprinter. They are super durable, so you wouldn't have to worry about it springing a leak under there
Aluminium would be light weight potentially not food safe, will corrode especially if you use roads which get salted or spend time near the coast.
SS would be expensive and heavy but would be durable and food safe.
Plastic would be my first choice personally. You can fit 12v heater pads to them if you will be using them in very cold conditions and are worried about freezing.
Not sure where you are in the world sass matt - I know some reputable suppliers in the UK but I'm sure you can find some local to you. If not then find any company that does plastic welding and give them the dimensions you want. A box should be about the simplest shape for them to fabricate so it shouldn't be crazy money.
Not sure what size you are looking for. I added the 15 gal version of this under my van as a grey tank directly under my sink. https://www.amazon.com/Class-Custom...coding=UTF8&psc=1&refRID=RNPB5X0J3RZW79G53BMY
They are the same size just longer/shorter in length. At 13" width, if fit perfectly snug between the rocker panel and the frame rail. I put mine on the drivers side next to the fuel tank. It sits just 2" below the floor because there are cross beams every two feet or so.