external power supply on conversion van

That sounds good - essentially "mechanical" switching using the OEM bits. Simple, quick, and solves your issue. Yep sure, boosting PC PSUs can come later - after all, isn't that probably the cheapest battery charger or 20A etc supply available? The next step - identifying pin-1 on ICs/chips (ie, where the dot is), therefore finding pin-x - in this case pin-2 next to pin 1, and tracing/mapping the copper PCB tracks from pin 2... Unless we can get a circuit diagram etc for that PSU (or a similar one). See you when you get back.

Ok I was able to do a test... I manually disconnected the bus from the ignition power source and threw the 230w supply that I modified last night on. It powered up my tv/digital box/90w inverter/cb radio. I didn't try anything else. Something I found interesting is with no load the 12v side showed 11.3v... when I hooked a headlight up to it to test it last night it raised the voltage to 11.9v. Today with all the other stuff hooked to it, it went only to 11.87v which would indicate the headlight used more power than the other stuff. The thing I don't understand is why when you throw a load on the 12v wire, the output rises. Its like the PSU is compensating for the higher loads which is great but odd. Since this smaller psu worked so well, it will become my conversion unit and I'll save the 300w for something else that has higher demands. Just too bad i don't have time to make this permanent before this road trip. :( Either way I'm happy to have it figured out. I might not even put a tv in the camper now. How can I get better than leather captain chair seating and stereo sound already built in. Keep the fireworks down until I return. :)

Well I'm back and finished wiring everything up. The good news is everything works. The bad news is that the tv doesn't work well. It appears to want more voltage. I say this because the audio breaks up when running off the psu but is fine when running from the van while its running. It breaks up when on battery without the van running too. Back to the psu project. :-)