Would help if you complete a signature which would state your model. That said, as stated, crawl under and locate two plastic hoses with screw on caps. One will be on a red line, one on a blue line. Take the caps off (they unscrew) and allow the hot and cold lines to drain. Open all your taps so that air can enter the lines as the water drains. Find the
inside access panel to your hot water heater and turn the valve which will take your hot water heater out of the loop and isolate it. Go outside and drop the water heater access panel. Take out the plug in the hot water heater and drain the water out of it. Go back under the trailer and find a plastic hose sticking down that is all by itself. That is the drain to your fresh water tank. Unscrew the plug and drain it. After all the water is drained reattach the plugs in the hot, cold, and fresh water lines. Now find the access panel to your water pump. On the take in side of the pump attach a long pliable tube. Put the end of that tube that is not attached to the pump into a bottle of the pink RV fluid and turn on your pump and put the pink stuff everywhere in the lines EXCEPT the hot water heater, which you isolated when you turned the valve mentioned earlier. Be sure to flush the toilet several time so that pink stuff gets into the flush valve. Some will even prop that valve open when they are done as added safety measure. To do my trailer takes about three gallons but I always err on the side of caution and use twice that amount. This video is helpful.
There is nothing to it.