I don't open the water heater temperature/pressure relief valve when filling the water heater.
Instead, I open a faucet.
If you open the water heater temperature/pressure relief valve when filling then make sure allow the water level in the water heater to fall below the level of the temperature/pressure relief valve before closing that safety valve. Both Suburban and Atwood RV water heaters are designed to have air pocket to accommodate water expansion as it is heated.
Toilet valve would be the flush valve on the toilet.
It seems lot a lot of info all at once but it all becomes more clear very quickly. Keep asking questions - everyone gets a chance to learn that way