Forgive me but I’ve used the search feature and read write ups until my head hurt and I’m still not sure what to do.

I bought this truck 3 years ago. The previous owner had pulled both the XREAS and RAS and swapped it for Bilsteins and FJ Springs.

I noticed a bit of sag anytime I put a lot of gear in the back. It wasn’t terrible but it was noticeable. I recently bought a 2500 lb Casita travel trailer and I’m not happy with the rear end. It sags too much and a noise comes from the rear until I hit the off button on the ride height. I’m assuming it’s trying to inflate the nonexistent air bags.

1) should I upgrade the entire suspension?
2) is there a modification I could do to the rear end shocks or springs?
3) can I add RAS with the current set up?

Thanks. And I apologize for my lack of suspension knowledge up front.