We traveled the River Road from the east end to Marisical Mine and back on a trip to Big Bend in March 2004. On that trip, that portion of the River Road was in the best shape of any of the primitive roads we drove in the park. We took the Jeep, but I don't think we ever had it in four wheel drive. It was much easier driving than the Old Ore Road, but I suspect that varies with how wet it has been, the last time they scraped the road, etc. I wouldn't necessarily count on it always being in that good of shape.
It took us about an hour to get to Mariscal Mine. We placed a geocache at that location,
GCHZ8D There are some links on that page to pictures we took on the trip. (Scan down to below the cache description but before the log entries and look for the little camera icons.) We had a blast exploring the mine and highly recommend it if you take the River Road.
On that trip the park was jam-packed full, at least by Big Bend standards, but there was plenty of solitude out on the River Road. Having said that, it was not as deserted as we might have expected. If you look closely in the one picture from Mariscal Mine, you can see multiple cars in the parking lot in the lower left.
The River Road would be an interesting trip, but five days sounds a bit long to me unless you are building in a lot of side excursions. I have never done it, but I would love to hike the
Mariscal Rim Trail
Good luck. Let us know what you decide.