Since nobody jumped in I will again.
I have tried a few things including installing a switch in parallel on the factory fan (which is a very noisy fan) to installing aftermarket fans.
The fan made by Valterra is about the easiest to install. I believe it is what ETI uses on the Texas fan option. It is not a high quality fan but for $32 I figure I can replace it if it fails.
It can be installed in the lower opening with a few mounting options. It comes with a switch and a temp sensor which I bypassed so I had full control of it. Since I had already bypassed the factory fan and had the fridge removed I installed it near the roof vent. I have found the lower factory fan actually works better which makes no sense to me but have also read in some literature from Dometic to install it lower below the condenser . I have a friend on the forum that ETI had a RV shop install the same fan in the lower opening and it seemed to work Ok but it was a older style 6.7 fridge which seems to have more problems then the 4.3 I have.
I thought about installing one of these solar vent fans but the reviews are bad on them and after looking at it they are kind of ugly so I returned it and I went with the other one. They are much larger then the stock Dometic cover too.
Amazon.com: Camco 42165 Refrigerator Solar Roof Vent System: Automotive
Dometic also makes a couple vent kits. I installed one in our Casita and it was very noisy but did work good.