Never heard of a manual dockside pumpout before!
However, your problem can be solved fairly easily, if not necessarily inexpensively. There's only one "kicker"...Install a macerator in your onboard pumpout line...the same kind of pump used to dump tanks at sea. Put it very close to the tank (but don't attach it to the tank), so that it primes quickly. Since the macerator is 1.5" in, but only 1" out, and your deck pumpout fitting is 1.5", you'll need a foot of 1" hose and 1" x 1.5" reducer unless you want to replace your pumpout fitting--which you don't.You're also gonna need a length of hose that reaches from your deck pumpout fitting to the marina's tank inlet (without seeing it, I have NO idea how you're gonna connect to it), and a fitting on one end of the hose that threads into your deck pumpout fitting...you can get that from SeaLand (not necessarily inexpensively though).Attach the hose to your deck pumpout fitting...stick the other end wherever it has to go...and turn on the pump. The whole project will cost you about $200--which may be the "kicker" to you, but this is one I was referring to: You're gonna have to find a place to store that hose, which will inevitably become a wee tad (?) "ripe" in the summer heat (you can keep that to a minimum by making sure to rinse it out VERY well after each use).Until your marina puts in an electric pump, a more affordable Alternate Plan B may be to just seal off the tank and replace your head with a portapotty you can carry off the boat and dump. The 5-6 gallon models hold as many flushes as a 12-15 gallon holding tank connected to a manual marine toilet, 'cuz manual marine toilets use so much more flush water. Alternate plan c: tell your marina owner that he can get a federal grant to pay for a REAL pumpout.  Plan D: move to another marina that has one.