I have a tank and a gravity system but drain it for winter. It seems like a maintenance nightmare unless you are somewhere that it never freezes. I bought my tank through Farnum, but I don’t think they sell them anymore. It’s tall and triangular so fits into the back compartment of my slant load. It’s 30 gallons. No pressure behind it but handy for drinking water and sponging off your horse if you need to.
You can get 12V sprayers with 25 gallon tanks at places like harbour frieght. I have one for my four wheeler that I use for spraying weedkiller. It would certainly work for horses. You would have to figure out where to mount it. I rigged up a plug for mine so that it would wire into the four wheelser battery.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90679
The heat is another issue. You would almost have to have propane as the electric required would drain your battery too much. You would have to look at an RV place to get one of those instant on heaters.
Personally, if it’s cold enough that you need warm water you probably shouldn’t be hosing your horse off anyway.
Anyway, you certainly don’t want more than 25-30 gallons due to the size and weight. 30 gallons of water weighs 250 lbs. Thats more weight for your trailer and tow vehicle.
This is an interesting topic, I’d be curious to know what you decide. Keep us posted.