The trickle charge is actually a function of the boat's on-board 120v power system when I'm hooked up to shore power. The Seward Small Boat Harbor has enough folks that pull their boats out for the winter (mostly 6 pac fishing charter boats) that they graciously allow us a winter slip with power during the off-season. We paid a deposit on the electricity and pay a monthly meter charge (usually about $20/month) and that keeps the batteries up and allows us to camp out on her over the winter as a sort-of winter cabin.
I did look up the specs on the pink RV style antifreeze and it is used full strength. Can be diluted but looses some of it's effectiveness. I think the frozen head had to do with the head intake seacock not being fully closed. (I discussed that in another thread).
It is with some trepidation that I look forward to seeing how she fared on the hard this winter...I may have to replace the batteries if they don't take and hold a charge in the Spring. I wish I had taken time to install a solar powered trickle charger to keep them up over the winter...oh well, water under the bridge, I suppose.
BTW - I talked to my wife yesterday and she said there was snow on the hills about 500 ft in elevation above town. Looks like it might be a heavy snow year back home.