Look for a suction port very close to your water pump. It will likely be a clear poly length of tubing which tees into your pump plumbing. It will have a screw-in plastic plug.
I made up my own extension:
I removed that plastic plug and made a trip to Lowe's. I purchased a threaded adapter (same threads as the plug) which I could slide the length of tubing onto, and secure with a stainless clamp. I use that "extension tube" only to add RV antifreeze, then replace the plug.
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