You need to drain your hot water heater and operate the bypass valves near it.
By-pass valves operated stops RV Anti from entering the water heater also make sure you shut off electic heating element if you use it.
With the water drained what ever is left will not matter just leave drain hole open.
The rest of your plumbing system pump RV Anti through it to protect your toilet valves and water faucets.
Place Anti in your traps, sinks, shower and tanks so it protects your dump valves.
You can
adapt this thread for what is needed to be done in your trailer.