Water storage tanks, either original or replacement, are not designed to handle dockside water pressure. Do not pressurize your tanks. Part of the purpose of the tank vent is to prevent over-pressurization of the tanks. Storage tanks are typically built to handle less than 5 psi. Dockside water pressure would be at least 40 psi, probably closer to 60 psi or more.
I have similar valving on my boat. With it, I can (carefully) fill the tanks OR, by bypassing the tanks, pressurize the house plumbing. While theoretically I could pressurize the tanks if the valves were set incorrectly, I would quickly regret doing so. Especially if they were not vented.
Your tank is certainly vented somewhere. The water that you use at the sinks has to be replaced by air as the level in the tank goes down.
