I can't help you find the screw holes...
They may have been a factory installation, or they may have been owner installed. As for any tank mounting fittings, it's unlikely there ever were any, except for straps. The v-berth location is fine for the tank, provided it doesn't put the tank further than about 6' from the toilet. And it's ok if it's slightly above the toilet...most tanks are. But instead of suspending it, find a tank that'll fit the space. Ronco Plastics has over 100 non-rectangular shapes, including "bow" shaped and triangular wedges to lie along the hull. Their catalog is on their website at http://www.ronco-plastics.com Specify your fittings locations to put the inlet from the toilet and the vent fitting toward the centerline of the boat, away from the hull...that's to prevent waste om the tank from running out the vent and/or back toward the toilet when you're heeled. Now for the downside to your plan: Because portapotties use so much less flush water than marine toilets, you'll need at least an 15-18 gallon--20 would be better--tank to gain any real capacity advantage (the number of flushes each will hold) over a 5-6 gal. portapotty. You'll be trading off a system that has almost no plumbing for one that has a seacock to worry about keeping closed, a toilet that must be maintained and kept lubricated, odor in the head from sea water trapped in the head intake, hoses that can permeate, and loss of storage space to house the tank. If you stick with a 5-6 gallon portapotty, permanently installed and fitted for pumpout instead of one that has to be carried ashore, you'll have all the convenience of a marine toilet and holding tank with none of the problems...and save at least $300.
