You just need a 12VDC LED PWM dimmer switch. Many are available. Search Amazon or SuperbrightLEDs.com. They are all pretty much the same functionally. Choose one that matches the aesthetics of your van. There are RV-style ones available.
There are two issues:
1) Most of these devices control the NEGATIVE lead to the LEDs. There is a pretty good chance that your coach is wired with the switch on the POSITIVE side, which won't work. If there is no negative return wire (i.e., if it uses a chassis ground), the job gets a little more complex.
2) If you need or want a 3-way switch (e.g., if you want the dimmer by the bed and also another on/off switch at the exit door), you need a switch with this capability. No big deal, just different.
There are also inexpensive dimmers that are controlled via a little IR remote. I don't particularly like this style, but it is an option.
If you need more help, give us details, including a picture of your current switch.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)