There is a rest area and nice state park at mile marker 45 on I-5 between Medford & Grants Pass. I don't know if the rest area has water but the state park would. Not in Oregon but in Texas and AZ I have stopped at RV parks and asked for water. Some were free and some charged so I suspect you could pull into any RV park along the way and ask. Why not avoid the concern and fill up at Lake Siskyou Resort? I bet they have good mountain water.
Bob, Sandi & Marmaduke the Big Pug
SW OREGON 2004 Journey 39K, 330 Cat
If towing: a 2006 Mini Cooper or 1995 Wrangler