Here's a pic of my version of the tool that KIX described. I happened to have some 1/4" copper water line left from an automatic ice maker project so all it took was the proper plumbing fittings to go from a water hose connection to the copper.
I just used a pair of pliers to bend and crimp the end to make it spray. These are also available from Camping World in a plastic version. My theory has always been if I could buy it for $5, I could probably make it myself for $10. Anyway, this works for me.
About the vinegar quantity, I think I would flush well first and then if I thought it still needed more, I would start with a gallon, let it sit and then flush again. Ten gallons of vinegar is a lot and would be my very last resort.
Denny, Connie & Shadow (former barn cat made good)
2000 Itasca Horizon, Cat 3126B, Freightliner Chassis