Wombat,
Just a small point, but within the wider aviation world, RNAV stands for Area Navigation. If you search for, or buy books on, RNAV, you may not be getting what you think or need! I assume by RNAV, you really mean Radio Navigation using the VOR/ADF? I used T Thom and it tells you all you need to know.
BTW - RNAV requires a lot more equipment than found in a SEP - it is the ability to fly *any* track (not point to point) using whatever ground and satellite based equipment is available, to a defined level of accuracy (BRNAV- Basic RNAV 5nm, and PRNAV - precision RNAV 1nm).
If the skill test requires it, you have to do it, whether you think its a good idea or not - and be assured you *will* be tested on it - fail it and you fail the skill test!
Using a PC simulator is a very good idea to learn the concept - put it on autopilot and use the autopliot heading function to fly to/from VORs and ADFs - track inbound/outbound, intercept and follow radials etc. This will teach you how the radio navigation works (tuning, identification, OBS interpretation etc.) and how to orientate yourself. Put some wind in as well - try it with 5kts, 10kts, 20kts, and 40kts to see the effects.
When you get to practice it in a real aeroplane you will only have to worry about the piloting side, not what the instruments are telling you and where to go, as you will have found that out from the PC simulator.