I have an a/c powered compressor, you don't need a large unit, but I don't know if a d/c unit would be sufficient.
This is what I have done. I've taken the hose that comes with the compressor and gone to Lowes and adapted the standard end to be a female garden hose end, I also put a shut off valve at the end of the air hose so you control the flow or air at the end of the hose and not only at the compressor.
Go to www.marineeast.com and order the 'hose barb to garden hose' adapters in the sizes that you will need.
http://www.marineeast.com/a_sch/sch_det.asp?cid=01&pid=01_01
I bought just about every size adapter, 3/8", 1/2", 5/8", 3/4", 1", 1 1/4", 1 1/2". I've helped some dockmates and it's nice to have all the tools without worrying or jimmy rigging.
I blow out my A/C units, my saltwater washdowns, bait livewells, my entire fresh water system, including the water heater.
I leave my boat in the water, so I also blow out all of the seacocks as well.
To do this you shut the seacock, take hose end that is not attached to the seacock, attach the barbed adapter to the hose and to the compressor, let the pressure build up, then open the seacock, when you hear air blowing out of the seacock, you shut the valve. Move on to the next system.
You might have to take both hose ends off some pumps to drain them and blow out each side of the line, as some pumps do not allow air to freely flow thru them.
I use pink non-tox in the holding tank, heads, bilge sumps, washing machine, genset and mains.
Hope this helps.