last house batteries i bought i got from auto zone - just 12vdc deep cycles (biggest they had and i could fit).
if you're making your own cables i highly recommend this hydraulic crimp tool - its inexpensive and works VERY well - I've made at least 2 dozen crimps w/ it on large 0 gauge cables (car stereos etc), catch it on sale and you can buy it for about 39 bucks -
Hydraulic Wire Crimping Tool
how do you mean you have exposed water line? i have an "out door" shower - and we've been down to 20 degrees (low 40s during day) and never had an issue. I don't winterize but were in texas too.
our holding tanks are not insulated or heated - always thought about it, but we never see cold enough temps....luckily our storage is enclosed and rarely dips below 40 (even when its in the 20s and 30s outside.
i would suggest, installing some electric radiant heaters (i use some that are oil filled)....two of them can keep the coach 70* when its in the low 40s.
Its a good practice to open cabinet doors / bathroom doors, doors that expose under sink so the heat can circulate into those areas (where plumbing is)....and old trick is to stick an incandescent light bulb in the area & turn the light on....the bulbs throw off a fair amount of heat and can keep the area warm enough to prevent a burst pipe.
Can you install a shut off valve (inside) on that exposed water line (just the water off to the exposed line) and drain it....otherwise all you can do is insulate and maybe heat tape. If you don't use it or need it - id drain it.
i would also investigate the type of crimp (PEX) fittings that are used on your coach and buy some crimps and the tool used to crimp the rings/clamps etc....also might be good to have some shark bite splice fittings (so you can just out the leak and repair w/ new water line & shark bite fittings.