That is correct on the grey/black. What we did is capture most the grey by putting a tub in the sink then dumping outside when done washing dishes, etc. Still, I'd love to have a separate tank and may research it. I imagine someone has done the mod already. The access looks very straight forward as its open under the trailer. I'll grab some photos next week when I get the trailer again. I also was thinking about rerouting grey to another add on tank, if possible. There are a few IntechRV facebook groups where we got a lot of info from. People have done some great improvements.
The heater is very quite IMO and gets hot! It took a while to figure out the best temp. I need to experiment with using the heater with a fan on to spread the heat a little better. Its located under the fridge area. The AC is much louder. We haven't used the interior stove yet beyond testing, but looks to work fine. The water heater got burning hot pretty quick. You are able to use both battery and LP to heat it at the same time. I waited 20 minutes and it was burning hot.
Also, the fridge worked great. It's a 3 way fridge that can work on LP, battery or shoreline. The outside fridge also worked perfect.
Feel free to PM me with any questions at all.