My fresh water tank has a fill spout on the top, you fill it from the inside. There is no "outside" fill. And since we added the larger port (5" vs. 1.5") you can either pour a bucket directly into the tank, or even extend the shower hose and put the hose right in the fresh tank.
I like being able to pour from a pitcher, or bucket, instead of needing a funnel to use the 1.5",original fill port on top of the tank.
I'm mostly worried about the water pump... so if I run some hot water into the tank, then run some water in the shower, I should have both the pump and fresh tank safe. Or at least that's my reasoning.
The 5" marine port I added to the tank is really slick. I can fit my entire hand down in there and wipe out 99% of the inside of the fresh water tank with a bleach solution. And I can protect my low point drain by merely plugging the outlet hole from the inside of the tank with my finger, then having my DH open the low point drain to get that 1 teaspoon water that might be in it, out.
Anyway, thanks for the info. I had always wondered if anyone else thought about mixing hot water in their tank to help out with occasional winter-freeze issues. All the traditional winter fixes, i.e. heat pads, heat tape, extra-recirculating plumbing, while totally valid, seem pretty excessive for only needing it 3-4 nights the entire year - spaced over 3-4 trips.
