Did exactly that to the shower floor in the TC last fall, although it was for a little bit of warmth rather than as a shower mat. Works well to keep feet warm, but haven't showered in the camper since installing the liner. Our floor is smooth fiberglas but I haven't noticed it sliding , so I think your rough floor should hold it just fine.
Brian
2014 Chevrolet 3500 CC 4X4 Duramax, Tork Lift Tiedowns, TorkLift Fastguns, Superhitch and supertruss
2009 Jayco 213 SOLD
2008 Bigfoot 25C9.4