My first boat, back in the '70s, was equipped with a standard elsan 'bucket' which was state-of-the-art at the time. My present boat came fitted with a pump out holding tank with a very nice dump-through Mansfield toilet which I stripped and overhauled. Its staple diet (apart from sewage) was 'blue' as in my elsan days but, having read the documentation which came with the Mansfield, I decided to convert to a more biological method using a nutrient-based additive which was quite economical. Having had a thorough pump out and several flushes of plain water, I commenced using this nutrient solution (bought from Millar Marine at Shardlow) and it only took a couple of emptyings before it began to work effectively and the rather pungent smell of the 'blue' disappeared.
Since my plans were to cruise the system in my retirement and enjoy the solitude and peace and quiet of the countryside in both summer and winter I was concerned that the frequent pump outs which I anticipated from such intensive use of the boat would give rise to rather high running costs. Accordingly I wanted to combine the cheapness of elsan disposal with the convenience of a holding tank. I purchased a manually operated bilge pump, some jubilee clips and some hose and grabbed an old disused piece of Contiboard from the deep recesses of my garage. I also acquired a large plastic drum/bottle which originally held the detergent fluid used in commercial car wash machines. It holds about 25 litres which is not particularly difficult to lift and can even be walked short distances. I actually truck it from boat to elsan disposal in a Do-It-All folding plastic box on wheels. I originally bought 1" i.d. clear, reinforced plastic hose, which fitted the o.d. of the bilge pump outlet but this proved rather stiff and unwieldly so I have since replaced it with 3/4" i.d. hose, using short lengths of the 1" to join the smaller hose to the bilge pump. At 3/4" this is much closer to the inside diameter of the pump outlet. 30 strokes on the bilge pump handle will fill the bottle, and 4 bottles-full will empty the holding tank.
Not all BW sanitary stations are suitable for self pump out and some work with a cesspit or septic tank, but those connected to mains drainage are eminently suitable. Of course you can do without the bottle and wheeled truck if you simply purchase enough hose to reach from the boat's pump out point to the elsan disposal and thus be able to pump it straight to waste. The whole outfit cost me no more than £50 and I reckon it paid for itself within just a few weeks.