In order to help with your issue, we need to identify how you treat your tanks but for this problem the symptoms are often that you might have a clogged vent pipe, not be using enough water during flushing, or you are not being thorough enough about the cleaning.
If you just let it drain and do nothing else, you could be asking for trouble in the short run. If you do some cleaning, then it will be a little longer. It's very important to rinse out the black tank very, very well. If it's hot and there's no wind, the smell might be coming from the vent pipe exit at the roof.
After the thorough cleaning if you do not keep 2-5 gallons of water and "solution" to prevent "cementing" this could be your problem.
If you don't flush with enough water when you "go" then the contents can solidify and not drain when you open the drain. We use the 5/10 second flushing rule, others have used less and done okay. This will depend on water pressure but you must let plenty of water follow your bio visit to keep it liquified.
Many of these problems are caused by not knowing how to maintain your tank or not doing enough. Some don't but I do use a sanitary/deodorizer in my tank. If I start to smell that, then I know I'm not cleaning the tank correctly and prefer that smell over the sewage odor. Use a clear adapter at the trailer's sewer outlet to ensure water is coming out clear. This is NO guarantee but it's a help.
Describe what you currently do maintenance wise and maybe we can help with other ideas. Apologies to such details but we ALL do this so there's nothing to be embarrassed about.
Ah, that's a big difference. I have heard from owners on other forums of onboard washers this can happen if:
1.The washer drains into the black not a gray tank, or directly into the sewer outlet. Some have rerouted the drains as needed to fix this. This way you can leave a gray tank valve open and prevent a full tank busting because the washer drain water has no place to go. Just don't forget to close the gray valve when flushing the black tank. You don't want cross-contamination. Never leave a black tank valve open.
2. Whomever installed the washer drain lines did so with low points so some of this drain water remains in the hoses. Apparently this rinse water has enough detergent or other chemicals that causes a smell. I do not know how the venting is done for this but should be investigated. My rig has the W/D prep and I've seen a vent pipe for the drain but I don't know if this vents to the outside or inside or how high it goes. I won't worry about it until I install a W/D.
3. How is the venting done? Most owners I heard for using washer/dryers do not like internal venting. Apparently appliances can be purchases self-venting or external venting - I don't know much more than that. Maybe it's because it causes odors if not vented properly or something goes wrong. I'm not sure of the details how it pertains to either the washer or the dryer. I try to relate this to the stick house plumbing.
4. Drain pipe vent is too short. The vent pipe for a drain needs to be pretty high especially if the water pressure is high (you have a strong washer pump that evacuates the water at a pretty good pressure). Then if the vent pipe does not extend high enough the water might just back up into the vent and then spill into the rig. This standing water will be pretty stinky if it stays in your rig, not to mention the damage it could cause.
I hope this provides more ideas of what to check. Are there any other W/D users in RVs that can provide more insight?