<< Is the gray tank that shallow?>>
YES, they can be, many gray/ black tanks have a tapered bottom. I suspect the inside of my black is only 2" to-4" deep at the higher, shallow end, (where toilet connects) which explains why I ONLY have about 1-inch clearance around down-pipe from toilet, (end of pipe to bottom of tank) which also leads to frequent clog potential, and occasional real clogs. From memory, 6"-8" deep on drain end, tank about 18"-wide, ~6-feet long, (25-33-gal??). Suspicion is gray tank is VERY similar.
(TerryH.) 2000-GS Conquest Limited 6266 Class-C 99-E450SD V10