1978 V20 Steplift trying to find leak?

[Blue_Runner]

Welcome to the site. My first thoughts are that 8 - 10 gallons of gas would not cause that kind of drop in performance. Water weighs in at 8 lbs per gallon so even at 10 gallons that is only 80 lbs.

Trying to find the leak - If it were me I'd probably get down under the boat and do a good visual inspection looking for cracks or gouges. Not sure of your configuration on the 78 but you might let the exterior dry completely and fill the plugged bilge with 10 - 20 gallons of water and see if you notice any dripping out around the thru hull fittings or drain plug. It wouldn't hurt to put some marine sealant around all fittings that are below the water line while your at it. The next time you take it out you can see if it made a difference.

About the tank being unhooked - where is it unhooked? Can you ask the previous owner why it is unhooked?