No power to anything after I installed battery backwards

Start by checking all the fuses. There's going to be many that are blown.

The first drawing shows the location of the Integrated Power Module, (under-hood fuse box). Use an inexpensive test light to start checking the smaller fuses. Each one will have two tiny test points on top. This article shows how to use a test light, if you need it:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

You can find these at Harbor Freight Tools, Walmart, or any hardware store or auto parts store for around $5.00.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Don't get excited if you have a charging problem once the engine is running. That would likely involve a fuse link wire, but those typically take more than two seconds to burn open. We'll address that later if it becomes necessary.

The smaller cartridge fuses can only be checked visually.

I don't see any reference to a fuse box inside the van, but you might want to check under the steering column area.