Basically, there two types of pilot ignition ...one is manual and the other DSI (direct spark ignition). On an older RV like yours, you could very well have a manual pilot where you actually have to light the pilot manually. With a DSI, it's just a flip of a switch.
As others have said, check your documentation as to the type you have.
Let's assume you have a manual pilot. In this case, you'll need to light the pilot using a long match or a striker tool. Again, your documentation guides should give instructions on how to do that.
Most RV water heaters are either Suburban or Atwood. If you do have a manual pilot light, the procedure should be similar as to how to light the pilot.
If you can't find your user guide to the water heater, here's a video that shows how to light a manual pilot light on a Suburban water heater that might help. If it doesn't, like others have said, post the make and model of your water heater and maybe we can help you find the owner manual for that particular model.