Pick a slow day at the marina and fill and pump the holding tank several times.
As I've posted many times, flushing out a holding tank does not require filling it. Put water into it via the deck pumpout fitting--'cuz that sends the water into the tank at the bottom to stir up any sludge so it can be pumped out---to a depth of 6-8"....pump that out. Repeat, repeat...till you're pumping clean water. If you have macerator pump, add a few gallons of water once more and turn it on to rinse out the macerator and it's plumbing. This should be done at least 2-3x/season and especially as part of prepping for winter or other extended layup.
Now you're ready to replace hoses.
If this is your first time replacing hoses, a couple of tips that should prevent it from being a nasty stinky job:
Start with the highest connections, duct tape the ends as you remove each one.
Warming the old hoses a bit (I always used a blow dryer) will make them easier to get off the fittings.
Put a plastic waste basket liner under each connection to catch any spills.
Warming the hose also makes it easier to get the new hose onto the fittings. Lubricate the inside of the hose and the outside of the fitting with a little K-Y...it's a water soluble surgical jelly that dries out and is also much slipperier than dishwashing liquid.
And I'm sure you already know that all hose connections should be double clamped, with screws 180 degrees apart...or at least 90 degrees if access makes 180 impossible.
--Peggie