Mine are easy to see too. The gray tank is near the rear under the toilet. The gray tank for the shower, bathroom sink and outside kitchen is in front of the black tank. The gray tank for the inside kitchen is in front of the other gray tank, which puts it over the axles.
2013 Silverado 2500HD LTZ CC 6.6L Duramax Diesel
2014 Sunset Trail SF270BH
Holly & Buster, mini Dachshunds
2016: 1 trip, 3 nights in 1 state, 316 miles
2015: 5 trips, 18 nights in 3 states, 2707 miles
2014: 5 trips, 14 nights in 3 states, 2426 miles