SurePower 1315-200 battery separator installed here. About $130 at the time I bought it 2+ years ago.
This is the second one - the first one began to operate intermittently after about a year of use. The solenoid in the unit would throw and attempt to close the connection, but the contacts (which are supposedly silver alloy or plated and should be less likely to corrode) wouldn't always form a connection (intermittent)
The unit was technically out of warranty, and SurePower customer service was reluctant to replace it - I eventually got ahold of an engineer, who couldn't believe that it had failed in the manner that it had and agreed to replace the unit if I would send the old one back in, so that he could take it apart and analyze it.
I believe Turtle has experienced similar issues with his unit (same brand and model number)
BTW, I don't believe an isolator will work on a Sprinter, due to the type of alternator used.