I'm looking for a nice multi piece rod to keep in my truck what would be the most versatile rod for trout, panfish, and river bass. I'm considering a 6'6" ML spinning rod with a 1000 or 2500 sized reel.

If I were in your shoes I'd get the fenwick methods. You can make a medium or medium heavy, should be able to chance the action from fast to extra fast too. I've had 3 fenwick rods, and they are all pretty nice. Perfect for travel!

I have a Diawa Ardito M/F casting rod paired with a Diawa T3 Ballistic and an Okuma Citrix ML/F spinning rod paired with a Pflueger Supreme XT I keep tucked behind my truck's back seat.  The Diawa is typical Diawa fit & finish and mates perfectly with the T3.  The Citrix has a nice tip I can throw light baits with.  Both come with semi-hard cases.

I have 3 travel spinning rods: 2 Cabela's XML (1 M 7' and 1 ML 6') and 1 Cabela's Salt Striker MH 7'. All are good rods.  The XMLs are 4 piece, the salt striker is 3 piece.  The best all around is the XML 7'. I use a Penn Battle 3000 on it.  Over kill, but very versatile. XMLs are no longer made, but there are plenty of rods of equal quality out there.  You might also find XMLs on eBay.   The 6'6" is great, so much so it is my full time drop shot Rod   I use an old 2000 size Daiwa team pro on it with 10# Sufix 831.  

My 4th travel setup is a Fenwick Eagle 6'6" casting Rod in 2 pieces.  I have a travel tube I bought cheap from Cabela's on sale into which it fits nicely.  It's a MH, with a Lews on it.  Just used it last week on a road trip.  Really performed well. It's a little long for an air trip, but fine for the road. 

I have a Shimano Exage. Its a 4 piece 7ft medium. The action is really nice and sensitivity is surprisingly good for a 4 piece rod. I keep it in my work van but I also use it regularly.