I've got an RDS 40 gal. tank w/toolbox combo in my 2018 3500 DRW. Originally it was plumbed as a gravity feed into the OEM fuel inlet hose to the OEM tank. I'm still using the same plumbing, but put in an inline fuel pump instead of using a solenoid valve for gravity feed.
I've had this setup for several months with no problems or codes/messages.
2018 3500 DA/DRW 4x4 LTZ