Putting an inverter in a G-20 van.
 

I have a 94 G-20 conversion van. I use it for night fishing and for work to do small carpentry jobs. A 500 watt inverter was given to me for free and I wonder if I should run the positive wire to the positive battery post or to the alternator. It will have an inline fuse holder for protection. I will obviously only use it when the motor is running. I'm also considering upgrading to a 100 amp alternator because I plan on having a secondary battery to run my cpap when I go camping and to power the inverter.

Consider separating the engine and coach batteries. That way if you kill the coach battery you can still start your engine. :) Google for ACR, battery separator, battery isolator. Lots of good info on there about doing that.

Alternatively make sure the inverter cuts off before it kills the battery; lots of them have settings where they will shut down if the battery voltage drop below some preset, usually 12.1 or 11.8V

It doesn't matter much with a 500W inverter. Just use whatever is most convenient or provides the shortest wire run.
Personally I would check if there's an aux stud in the underhood fusebox, just because I don't like extra bits hanging off my battery post.

there is no underhood fuse block and now I have another problem. I worked all day to have 2 new cigarette lighter plugs got the whole Fing thing put together with a plug that plugs into the old cig lighter and there's no power. I've pulled every fuse.

You need 50 amps for that many watts, and about 8 or 6 gauge wire. You wont get that from a cig lighter. You need to go to a 12 positive post add an inline fuse and go from there.
My 750 watt inverter only gives about 150 watts when it is connected with its cig lighter plug, but add the heavy clips to the battery and I get the full 750.

The cig lighter and the inverter are 2 separate issues. That is why I asked if I should wire it to the alternator or the battery.

cig lighter jacks work now. had it wired backwards and blew my mp3 adapter.

Already have a relay for that. Still need the $$$ for the battery and tray.

50 amps for 500 watts?

Current = Power / Voltage
= 500 W / 12 V
= 41.7 A