How to Fix RV Washer P-Trap Dry Out and Stop Gray Tank Odors

Bottom Line Up Front

Glenn, you're absolutely correct about how P-traps work in your 2025 Bay Star 2826. Without a washer installed, that P-trap will indeed dry out and allow sewer gases from the gray tank to escape through the drain pipe. The solution is simple: either pour a small amount of water down the drain monthly to maintain the trap seal, add a small amount of RV antifreeze, or install a mechanical trap seal or cap. Since Newmar pre-plumbed this connection, they likely expected most owners to install a washer, but there are several easy ways to handle the unused drain without any major plumbing modifications.

Root Causes

The P-trap in your Bay Star's washer drain connection works exactly as you described - it holds water to create a barrier between your living space and the gray tank's sewer gases. When water sits in the curved section of the trap, it physically blocks gases from traveling up through the pipe. This is the same principle used in all household plumbing fixtures.

Without regular water flow from a washing machine, several things happen to compromise this seal. First, the standing water in the P-trap will slowly evaporate over time, especially in the dry conditions common in many RV camping locations. Second, if you're in freezing conditions and the water freezes and expands, it can crack the trap or push the water out. Third, negative pressure from other drains in the gray tank system can actually suck the water out of an unused trap, a phenomenon called "trap siphoning."

The gray tank connection is the real culprit here - it contains decomposing waste, soap residue, and other organic matter that produces hydrogen sulfide and other unpleasant gases. These gases will naturally want to escape through any available opening, and an unused drain pipe provides a direct path into your bedroom closet and living space.

Before You Start

Before addressing this issue, locate the exact drain connection in your bedroom closet and verify it's actually connected to the gray tank system. In your 2025 Bay Star 2826, the washer drain should be a standard 1.5-inch ABS or PVC pipe with a visible P-trap underneath. You'll want to have a flashlight handy to inspect the trap and connections.

Gather these inexpensive materials depending on which solution you choose: RV antifreeze (propylene glycol based, not automotive antifreeze), a measuring cup, and possibly a drain cap or mechanical trap seal if you want a more permanent solution. Check your owner's manual or contact Newmar to confirm the specific drain configuration, as some Bay Star models have slightly different plumbing layouts.

Consider your usage patterns - if you're planning to eventually install a washer/dryer combo, a temporary solution makes more sense. If you'll never use the connection, a more permanent seal might be worth the small investment. Also think about your camping habits - if you're frequently in freezing conditions, you'll need a solution that won't be damaged by ice.

The Repair Process

The simplest solution is to pour about one cup of water down the drain every 3-4 weeks to maintain the P-trap seal. This works well for regular use, but you'll need to remember to do it consistently. Mark it on your RV maintenance calendar along with other monthly tasks like checking tire pressure and testing smoke detectors.

For a more reliable solution, pour about half a cup of RV antifreeze down the drain instead of water. The propylene glycol won't evaporate as quickly and provides freeze protection. This solution can last 2-3 months depending on conditions and only costs about $1 each time. Never use automotive antifreeze, as it's toxic and can damage RV plumbing seals.

If you want a permanent solution, install a mechanical trap seal specifically designed for unused drains. These devices cost $15-30 and create a physical barrier that opens only when water flows through. You can also simply cap the drain pipe above the P-trap with a standard pipe cap and appropriate cement (ensuring primer is used for PVC and that cement is compatible with your pipe material), though this makes the connection unusable if you later want to install a washer.

For winter storage or extended periods of non-use, always use RV antifreeze rather than water. Pour it slowly to ensure it fills the entire trap, and add an extra half-cup to account for any evaporation. This prevents both odors and freeze damage to your plumbing system.

Professional Help Needed

This is definitely a DIY-friendly issue that doesn't require professional help in most cases. The solutions are simple, inexpensive, and pose no risk to your RV's plumbing system. However, you should consult a professional if you discover the P-trap is cracked, if you smell sewer gases even after maintaining the trap seal, or if you want to permanently modify the plumbing system.

Contact Newmar customer service if you're unsure about the specific plumbing configuration in your 2025 Bay Star 2826, as they can provide model-specific guidance and confirm whether any recalls or service bulletins apply to your unit. A mobile RV technician might be worth calling if you're uncomfortable working around plumbing connections or if you discover any leaks or damage during your inspection.