Neil,
About 2 years ago I attempted a start and it was sluggish.  I pushed the jumper button and got started.   The mission for the day was a refer recall.  While I was at Sumner RV getting the recall, I got a quote for Interstate chassis batteries.  I was impressed but not the right way.  I got on the phone and found Budget Batteries on Old 99 in Milton.  (7900 Pacific Hwy E Milton Wa 98354  
http://www.budgetbattery.com/locations1.phpIt's big, for a battery place.  They have everything from fork lift to golf cart batteries.   They have all sizes and shapes.  The price was good and they were a match for what what I had.   I bought them and they installed them for me in their lot.  
Here is the weird part.  There was no label on the batteries.  I asked about having the warranty date on them.  The manager opened a file drawer, pulled out some labels, punched the date codes and stuck them on.  Always curious, I asked why the labels weren't on them BEFORE they went into my motorhome.  He said it was because they never knew who might be selling them.  He said that as a "wholesaler" other battery brands might be applied.   As I was leaving, a van with "Interstate" written on the side pulled in to load up with batteries!  The Interstate guy had his own labels.  
I think they have a branch in Bremerton if you don't want to pay the bridge toll.