40 Gallon RV Fresh Water Tanks

Meet your road-trip MVP: a 40-gallon, food-grade fresh-water tank that slips right into your rig, stays rock-steady on rough roads, and keeps showers, coffee, and pups happily hydrated all weekend long.

A 40-gallon fresh water tank hits a sweet spot for many travel trailers, Class C coaches, and van conversions, large enough to stay off-grid for several days yet compact enough to slide under a dinette bench or between frame rails. Below is everything you need to know before buying, installing, or replacing one.

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1. Why 40 Gallons?

Balance of capacity and weight – full, the tank adds roughly 334 lb (40 gal × 8.34 lb/gal), light enough for mid-sized rigs to handle without suspension or axle upgrades.
3- to 4-day autonomy – at the typical 8–10 gal/day for two people (drinking, cooking, quick showers), you can boondock a long weekend before hunting a refill.
Common OEM size – many RV builders standardize on 40 gal shells, so replacement tanks match factory plumbing with minimal fuss.

2. Construction & Materials

Rotomolded or blow-molded linear-low-density polyethylene (LLDPE) – food-grade, BPA-free, NSF/FDA compliant. It shrugs off road vibration, resists algae growth, and handles temps to ~120 °F (49 °C).
Wall thickness ranges from ⅛- to ⅜-inch; thicker walls dampen slosh and tolerate aggressive strapping on rough roads.
UV inhibitors are not standard, mount the tank inside the coach or in a shaded bay to avoid brittleness.

3. Typical Footprints

40-gallon shells come in several “shapes,” so measure twice! (Crazy side note: we even have a 100 gallon rv water tank!)

Pick the shell that slides through your compartment door and leaves 2 in of clearance for plumbing and expansion.

4. Inlets, Outlets & Vents

Fill port – usually a 1¼ in barbed or spin-weld stub for gravity fills.
Supply outlet – ½ in NPT (sometimes ⅜ in) feeding the demand pump.
Vent/overflow – another ½ in NPT routed higher than the fill to purge air and prevent tank “burping.”
Custom spin-weld fittings are cheap insurance if the stock port layout fights your plumbing runs.

5. Installation Best Practices

Full-surface support. Rest the entire tank on ½-in plywood or HDPE, not just on cross-members, to avoid point loads and stress cracks.
Strap, do not bolt. Use 1½- to 2-in nylon or stainless straps with turnbuckles; tighten just enough to prevent bounce.
Allow expansion. A full polyethylene tank can grow ~3 % in every direction; leave ¼ in clearance on all sides and use flexible hose to the pump.
Isolate vibration. Rubber grommets on straps and neoprene pads under the tank muffle pump hum.

6. Sanitizing & Routine Maintenance

Quarterly bleach flush: Mix ¼ cup household bleach per 10 gal of tank volume (1 cup for 40 gal), fill, slosh, sit 4 hr, then flush until odor-free.
Inline strainer: Install a 40-mesh screen ahead of the pump, cheap protection from shavings and debris.
Winterize: Drain completely or blow out with compressed air; a cup of residual water can split the shell at the first hard freeze.

7. When to Replace

Replace rather than patch if you spot:

Spider cracking around fittings, indicates polyethylene fatigue.
Bulging floor or creases, signs of repeated over-tight strapping.
Persistent plastic taste even after sanitizing; UV damage has likely compromised the resin.

Upgrading? A fresh tank with extra ports lets you add a galley-dedicated supply or a gravity-fed recirculation loop for hot-water pre-heating.

8. Quick Spec Checklist

Capacity: 40 gal (±2 %)
Dry weight: ~33-35 lb
Material: LLDPE, BPA-free, NSF/FDA
Max service temp: 120-130 °F
Standard ports: 1¼ in fill, ½ in supply & vent
Warranty: 1-3 years (varies by manufacturer)

Bottom Line

A 40-gallon fresh-water tank delivers boondocking freedom without overloading your chassis. Choose a footprint that fits, mount it with room to breathe, and sanitize on schedule - your faucets (and your family) will thank you on every mile of the trip.

Shop our 40 Gallon RV Fresh Water Tanks