Thanks for the quick response Greg. First off I must say that if after 3 years that you haven't changed the thermostat yet, that there is no reason to at this point. But enough about you, as promised I am ready to tell the fix for my Venture. BTW, thanks goes to you for your fuse/relay block photos. That was a huge help, and I should have thought to consult my owners manual first, but you had the pretty color photographs.
Back to business; Just as I suspected, the fans running on high was the problem. With the engine running, I removed the #9 5 pin and #10 4 pin relays to shut the fans off. I then felt the lower radiator hose start to warm up. With a smile on my face (smiling is optional) I sat in the drivers seat and waited for the temp gauge to start registering. A couple of minutes later, VOILA! The needle moved to the normal operating location. I had full heat from the heater again and I knew damn good and well that all was right in the Venture world once again. I reinserted the relays and the fans stayed off. I took the parts I bought from Auto Zone and returned them, told them of their blunder with not mentioning the fans, and asked them to reset the PCM P0128 code. While they gladly refunded my $$$, they denied my request to clear the code. Something about they aren't supposed to and it will clear itself in a few days of driving. So we will see, if not I will get my own scanner and that way I won't have to take the van anywhere when the CEL comes on again.
The main reason I avoided swapping the T-stat (besides the nasty location) is because in all my years of working on cars, and all the cars I have owned with T-stat problems, I have NEVER had one be stuck OPEN. Closed yes, but open, never. I don't know how one could be stuck open. Leaking very possible, open is not from my expanded experience. Anything is possible, which is why I entertained the idea but being stuck open is so very unlikely.
With this in mind, lets diagnose your "symptoms" that makes you believe you have a faulty T-stat.
I also have a lot of "physics" evidence that the thermostat is not functioning properly, such as the temperature gauge that varies when I am going down a long downhill stretch of road. The temperature gauge just continues to drop, even at relatively warm outside temperatures (in the mid to high 70's).
This is normal. When you are going down hill, gravity pulls the car and the engine is not working as hard. Thus, the engine temp will drop. Also with the lower altitude the engine will cool faster as the air moving thru the radiator is thicker and can absorb more heat.
And I can tell you, my engine does not stay at a stable temperature around 195 at all, unless I let it idle in a drive through line, or have a long uphill road to go up. But as soon as the van is moving through the air again, the temperature drops.
If you have a good cooloing system (clean, clear radiator, properly working water pump, good hoses with no soft spots, and working T-stat) the engine temp will indeed vary. Again, you altitude and location has alot to do with just how well your cooling system is working. You don't have any of that Dex Cool crap in your system do you? When the van is in a drive thru line it will heat up. Mine does. I don't use the drive thru for that reason and it is a needless waste of gas and wear and tear on an engine/tranny, and I can just as easily get out and walk in the place myself.
A perfect example was today when I let the van idle while waiting for the temp gauge to start registering. Once it was at normal, I still let it idle longer because I knew the temp would raise. As well it should. The fans were off (thankfully) and there is no air moving thru the radiator. As a result the temperature raised to the 1/2 way mark on the temp gauge.
After I reinserted the fan relays and I lowered the hood, I took the unneeded parts back to the store. Driving there, the temp expectedly fell to its normal temp. Nothing out of the ordinary here. Also, you can run your heater and the result will be a lower temperature due to the fact you are drawing heat from the coolant.
From what you have posted, the worse case scenario for you is a leaky T-stat. That won't hurt a thing as long as you are not leaking coolant from the goose neck or elsewhere from the engine. The T-stat would be the last thing I would be concerned with. However it is your van and obviously you know better than I the whole picture, and I am only basing my opinion(s) on what you wrote.
You never mentioned the fans being on or off in your posts, so you may not know either way. But I encourage you to look into it Greg, as this could be an added, unnecessary strain on the electrical system if allowed to continue needlessly. You can also check the temp sensor that, while easier to get to than the T-stat, is still a PITA to get to.
Finally, I have the same problems as you (and most others I've found) have;
-gas gauge screwed up. What I do is fill the tank and use the tripometer and when I reach 400 miles I fill it again and zero out the trip for another 400 miles.
-A/C filler valve leaks. I have filled the system a few times. When it works, the A/C works well. 3 days later it is like I never did anything. Dye in the 134A indicates the valve is leaking. I replaced the valve, still leaks. Never replaced the whole tube due to low funds. Should I do that, I cringe that the condensor will come up cracked like yours. I am not going to sink $700 into the A/C. We can live without it, hell, we have for the past 3 years
-The rear window motors barely operate and can't be used. I'm not going to spend $100 per just so the rear most windows can open. The sliding doors windows are fine.
So there you have it. I hope I have been able to help you and others with this cooling issue and P0128 code.