I have a
Pioneer in mine that I ordered from Crutchfield. I like Crutchfield a lot - you can find cheaper prices, but they don't include the installation kit that Crutchfield includes for free... so it usually works out the same, or even cheaper.
I didn't have any problems with fit like OC Eli mentioned - it went right in. The amp is hidden back there a bit but no problem disconnecting it. My only gripe is that I have to slide the engine cover back a bit to get to the screws at the bottom of the dashboard.
I also took the time to wire my radio up to the camper's battery, instead of the engine system, so that I can listen to it any time without having to run down the engine battery and put keys in the ignition. The only quirk from doing this is that it doesn't shut off when you shut off the engine. In mine, there is a large bundle of wires under the driver's carpet, so I connected both power wires from the radio and made sure they were both fused (that bundle of wires is NOT fused in mine).
(I then ran the stock power wire for the radio to the fog lights, so they are now fused and controlled by accessory power.)
This is probably the best thing I've done to the camper - many times, the radio is the first thing I turn on in the morning, and the last thing I shut off at night. I installed a Sirius tuner when I did all this. I'm not a huge fan of the HD radio concept, but that's because I can't stand to listen to FM stations any more - too many annoying advertisements, too little decent music. And you have to constantly find new ones as you're traveling. Sirius, on the other hand, is constant throughout the country (can you tell I love the Sirius?).
I replaced the two rear speakers with 5x7s I bought used off someone in a car club I'm in (the car in question also uses 5x7s). The fronts are still stock, but I probably won't bother replacing them. I only use the front speakers on the road, and then it's too loud in there to get quality sound anyway.
I took my CDs and ripped them to MP3 files, down to 192 kilobits (again, the noise from the van and all makes "high quality" not really possible), and I got one of those 12 CD holders for the driver's window visor. Each CD will hold 6-10 albums that way, which means I don't have to switch them often (although it works out that I almost always keep the CD with the albums from my favorite band in the player, and I use Sirius the rest of the time).
Edit: Moved to the In-Camper Entertainment forum.