Hot water tank bath

[BLTravels]

I am about to give my Atwood water heater a vinegar/water solution bath.
A couple of questions.
A couple of resources suggested not to take off the pressure relief valve to pour it in but rather use the country fill/winterizing tube to make it easier. How do I avoid the solution just filling up the FW tank? I obviously only want it to go into the HW tank. I don't want to find out the hard way I have lost all that vinegar. I can't imagine if it goes into the FW tank I then open a hot water faucet that I will get the whole six gallons out and into the HW tank. Plus I would then also have to flush out the whole system.
I am having a hard time finding large quantities of plain white vinegar. Will distilled do just as well?

Thanks

[muckinfuss]

I use a pump pressure sprayer through the drain hole. Vinegar...Tractor Supply, Rural King, just about any place that has a good supply of canning jars for sale....even Wally Word.