The smell should not be getting into the RV. Dump a quart of water down all of your drains weekly and keep ester in the toilet bowl. That may be your problem.
I've worked with livestock and there poop all of my life. It is good to keep solids out of your tanks. If you feel the need to wash them go ahead. But, you are never, ever, going to get them clean enough to smell nice inside. That is why there are p-traps and toilet valves with seals in them. But, if you could cut the tank apart and smell it inside trust me it to going to smell bad. Maybe not a lot if it is really clean but, it is never going to be that clean.
The pens in our hog barns have plastic planks separating them. We power wash the barn down with hot water between batches of hogs. If you take one of those plastic planks out and leave it in the open air for months it will still smell like hogs if you put your nose up to it.
Plastic will absorb some things. Maybe if your holding tanks were made of glass you could get them perfectly clean.