Should you fill up your on-board water tank before pulling out of the driveway or wait until you get to the park and use their potable water supply. The answer involves both personal preference from your own experiences as well as some good old fashioned physics.
Points to consider: The availability of water at your destination, your trailer & towing configuration, safety, and economy.
#1 Will you have access to potable water at your destination? This is very important to find out ahead of time! If you’re using a public or private park, they will almost always provide potable (drinkable) water. It may be available on site in full hook-up spots, or at a centrally located filling station. Sometimes the water fill is in the same area as the dumping station, which could mean a really long line-up on busy weekends. In some busier Ontario Provincial Parks additional dump station lanes are being installed, and even separate water filling areas. Don’t count on the water being turned on in the shoulder seasons and check for boil-water advisories or seasonal maintenance. We’ve encounter each of these situations and were glad we had water on-board. Don’t fill up with non-potable water, even if you plan to boil it, because it could contaminate your water system. If you are boondocking, find out if there is a public park or rest stop that allows fresh water fill-ups. Some RVers on long trips will stay overnight in a park or pay a day pass so that they can fill up and dump all their tanks along the way.
#2 How is your trailer configured? Some have shallow tanks that stretch across the bottom of the trailer while others might have a boxier tank located to one side or perhaps under the bed. Ours is located evenly across our axel. We find that if we’re traveling down the highway, or in windy conditions, a full tank provides a stability to our lightweight trailer. If your tank sits to one side, how is the remaining weight balanced? Where are grey/black tanks located if you have them, and other weighted items like a stove, pantry, or refrigerator? It’s always best to travel with an evenly distributed load. Some folks prefer to bring bottled water or large containers they set on the picnic table. Keep in mind, that load needs to be balanced, too.
#3 For today’s physics lesson: take a water bottle and will it half full. Put the lid on and tilt it back and forth on its side. The water starts to slosh around, just as it would in your trailer’s tank. This can negatively affect your steering around turns and handling up and down hills. With large enough tanks, your trailer may begin to sway, which is very dangerous. If you haven’t used all your water while on your trip, drain it on the way out of the park or at the park’s designated spot. Make sure it’s all drained before turning onto a highway since the spray can become a hazard for drivers behind you, especially bikers.
#4 Fuel economy. A gallon of water weighs 8.34 lbs at 62°F (3.78 kg @ 17°C). Our small trailer has a 22 gal water tank + 6 gal water heater. The water heater always remains full so that adds roughly 50 lbs plus another 183.5 lbs when the water tank is full. Topped up, that’s under 250 lbs. Heck, that’s just a trip to Costco with the kids! On short trips, this has very little impact on our SUV’s fuel economy but your situation may be different.
#5 Always, ALWAYS mind your tow capabilities. Keep your fully loaded weight within your vehicle, trailer, and hitch ratings. Towing over your weight rating is unsafe because it affects your steering and braking capabilities. A broken axel, hitch, or overheated engine are real safety risks on the highway and are a quick way to ruin a trip. Even if you plan to fill-up at the park, the water may be located quite a distance from your site, and sometimes up and down bumpy terrain.
#6 Is your water at home metered? If water fill-up is included in your park stay, you can save a little money on your home utility bill by using theirs. It’s probably not a big savings but it’s something to consider in your budget.