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got it. thanks for the info. the boat did come from salt water so I will check the strainer. I will also keep flushing the tank out and hope it gets better.

Yeah, salt water much more than fresh water will really stink things up. Just that little bit of water that stays in the head just stinks to high heaven when left. Another "tip" for a deep clean on the holding tank - pump it out, fill it 1/2 full of freshwater and a little bit of dishwashing liquid, then go for a ride and let it wash around in the tank. Then pump it out - make sure you are using a good chemical in the head. And last, make sure when using the head it get's flushed - I usually "flush" for at least 10sec, you need to make sure the "stuff" get's pushed into the holding tank - if it sits in the hose that is another source of smell. Sounds like a lot, but I have had boats with heads for years and never really had issues with smell by following these few maintenance items.

One more tip that I'd recommend, in addition to what billk2632 said, would be to make sure you never leave the holding tank completely empty, unless it is the last trip of the season. What I do for our RV tank is after I've emptied at the dump site, put in a RV toilet porta pack (found easily at Walmart or Amazon) and put that in the toilet, then run the flush for a good 10-30 seconds. This will leave clean, treated water in the tank while your boat is sitting. It will get sloshed around on the trailer or at your slip, and keep everything smelling fresh the whole time it's not in use, instead of stinking the entire time.