We changed our holding tank about three years ago. We gave up some storage but have never regretted it. My bet on your odors are old hoses. But you may have a clogged vent line. Especially if anyone ever tried to pump into a full tank.
We figured the location for the new tank and then bought the largest that we could fit. We had to tab in supports so we started with some closed cell pipe wrap and tabbed it to the hull to prevent any hardspots from forming. We then attached a frame to the tabbed in shelves and set the tank in the frame.
I had to cut a few pieces parts of the boat out of the way and splice them back in. But it was not too hard. I did the whole project on our mooring using cordless tools. Ripping out the old tank and dryfitting the new one and tabbing took about three hours the first day. Setting the tank and putting the boat back together took about two more hours.
It took more time to plan than to do. I wish we had replaced all our hoses then. They are on the list for this spring. Soak the ends of your new hoses in a pan of hot water and it will be much easier to install them. Double high grade hose clamps are imperative as well.
Hope everything comes out OK.