On the Thor motor coach site look under the owners resources. You input your serial # and it shows a list of fresh water diagrams.
My 2014 34e was there and I see a 34j but can't open it, I guess because it doesn't match my #.
Anyway, mine is under bed in a corner. I don't like to leave water in tank so I installed an access door to get to it wo lifting bed. Works great. I bought door at Home Depot. It is a 5x7 I think. I'll try to add pic but not sure it'll work from phone.