Congrats on the boat. Go to the Four Winns site at
http://www.fourwinns.com. At the top of the screen click on "Customer Service" (yes, it is in real faint gray letters. Click on "Catalogs", under model year 1995 click on "Fast Facts". Or click this link:
http://www.fourwinns.com/customer-service-catalogs.aspx and select your year, and "Fast Facts". This has all kinds of data and specs on your boat, original prop, engine choices, performance, fuel consumption, rpm vs speed, etc.
While you are there you can get an owners manual and access the on-line parts catalog.
The original prop was a 15" diameter x 17" pitch. With that prop the book figures for cruise are 27 mph at 3000 rpm, 32 mph at 3500, and 37 mph at 4000. Personally I would recommend going to the stock prop size and stay more in the 3000-3500 rpm range. Generally 1 inch of prop pitch equals 150 to 200 rpm. Less pitch= more rpm. I'm not sure on the Max rpm for that engine. The 7.4, like the other GM engines have gone through a metamorphosis from the early 1990s to today. Many changes have been implemented. The older engines were 4200-4600 rpm. The later engines are 4800 to 5200. I would recommend finding the exact model number and determine the specs for that engine.
As far as best fuel economy, I would recommend a fuel flow sensor connected to a GPS/Chartplotter. This will give you gallons per hour, miles per gallon, total fuel used since refueling, total season fuel use, or ??? My '02 268 with a 5.7 Gi seems to get its best fuel economy (2.3 to 2.4 mpg) at around 3600-3700. That is a little higher rpm than I expected. The worst fuel economy was when the boat was not quite on plane (2000 to 2800 rpm was less than 1.5 mpg).
The shower sump is known for having issues. Get out your Volt-ohm meter. Check the fuse. Check the wires at the pump for power. It is possible that the float switch in the sump pump is bad. I have changed 2 of them in the last 4 years on my boat. A new shower sump pump and box assembly can be bought for $100-150. If you want to dig into it, you might be able to find the float switch and pump locally, or mail order them. You might have a similar problem with the bilge pump. Make sure these pumps are operable. Hope this helps.