That old 1970 motor blues is why I went to the 90s for my SS, man I love that Merc 115 inline 4 cyl and it turned 30 this year. The V6 motors have all sorts of issues compared to the 4 cyl.
I saw up there your said "nasty Johnson" gave me a good chuckle.
Welcome aboard, the V5 designation was the early model of the Vs to come in the 80s and Starcraft was proud of them making emblems and decals to show it off. Another give away is the port side fuel filler.
Looks to be corrosion forming under the paint, not sure what that blue color is in the pitting but yeah it needs to cleaned up and stopped. Being on the bottom makes me suspect either wet with brine on the bunks or someone put on copper anti fouling paint.
The tanks were custom made to fit the belly. What you can do is remove the deck supports, fit your tank and then put the supports back in and attach them to the ribs on the outside edges of the tank. I did this with my Chief and then added height to the ends of the ribs to level the deck.
Moving towards the finish line and some nice carpet work, that stuff isn't easy to work with.
For sure you'll want to box up the batteries and anchor them down
I used 1/4" batting on my SS panels just to add some cushion for for the vinyl after seeing how easy it gets injured on my Chief with just the plywood underneath.