David - I did this in my 281, ably assisted by Sven! Not a pleasant job but well worthwhile - I replaced all the hoses and the tank itself - made a huge difference to the interior smell of the boat.
It is possible to replace the hose without removing the shelf, tank, etc but I must admit, I did do away with the y-valve and just ran a single hose direct from the head to the tank going through the locker and not back under the aft berth like the original hose did. I was never going to be far enough offshore to be able to dump the tank so figured the y-valve was just another added complication I didn't need. It was pretty fiddly to get the hose back up from the bulkhead through the corner of the shelf for the water heater but I got it through in the end.
In looking at other discussions, one other owner did same thing re. hose direct to tank, but added a vented loop as well.
Everything I read up about hose said don't go cheap - I posted the process I went through on changing out the hoses, etc on the google groups website
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There are also a couple of other discussions - search on Sanitation Hose.....
Good luck !
Al.