1. If you have a holding tank that has easy access where you can gravity fill it using a 5 gal jug, you can cut an 8" hole in the top and seal it using a matching size marine deck hatch. It screws in place and is a waterproof seal. I have done that and it also allowed easy cleaning of the inside of the tank.
2. Or...Install your own gravity fill. Run a gravity fill port and hose, like a pool hose. Run it to the top of the tank and seal it. Place the fill port in an appropriate place inside the rv, near the tank. Fill it by hand with a funnel or if you do not want to hold a 5 gal jug, use a bilge pump and a clean bucket.
3. use your outside overflow tube. If you have an overflow vent tube on your holding tank remove the screen and use that port to gravity fill the tank . Again use a funnel or buy a small bilge pump.
For 12v power, I use my positive and negative outside solar port to plug into to power any 12v device. Just buy the appropriate 2-wire connector. I use it for my bilge pump and for additional outdoor 12v LED lighting around the campsite, so I can keep the bugs away from my entrance door.