Welcome to JOF
Normally the black is close to (or under) the toilet, and the fresh tank(s) close to the outside fill location. The grey location can be a mystery at times but tends to be close to the bathroom.
A call to Jayco with your VIN handy can conform actual locations.
I found my tank locations (in relation to the axles) by removing the manual slide access panel under the TT and took a peek.
2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (integrated sway control)