I have a 1/2 bath in my gameroom where my tank is. My wife calls it my laboratory
I have my RO/DI unit mounted to the wall in there. Inlet and drain runs to sink next to it. Just on the outside of the restroom I keep a 65 gal. vertical container for mixing my saltwater. I run a long 1/4 tubing from the unit out to the container. I fill it with water then wind up the tubing and hang it back in the restroom on a hook. Next to my mixing tank I have an electrical outlet. I drop an internal pump into the container, plug it in and add salt.
After it mixes for for 24 hours I just let it sit until I'm ready to do a water change. On water change day I siphon water out of my aquarium with a hose that runs out my back door. To add clean water back to my tank I attach my aquarium hose to the internal pump and plug it in. The hose reaches from the holding tank across the room to my aquarium. I can do a water change in about 10 minutes. During that time I will clean my skimmer cup and sump. To make water changes even easier I bought some plastic containers that hold the exact amount of salt I need for the 65 gallon holding tank. I buy a couple buckets of salt and divide it up into these containers. That way I just have to grab one and dump it into the 65 gal. of RO/DI water with the pump running.
65 gal. holding tank
Pond Master 1200 internal pump (goes inside holding tank)
Python aquarium hose (screws onto internal pump for refilling aquarium)
PVC refill wand (made something like this to hang on tank and connect to Python hose so I don't have to hold the hose as I refill the tank)