From a forum post by DearMissMermaid on RVForum.net
My used Class C came with odors too when I bought it. I didn’t notice them when I looked at it, but after I bought it, yucky poo!
I started pulling everything out of the rig such as removable dinette cushions, mattresses, drawers and so on. Next came sweeping and vacuuming. Then I began cleaning with a simple solution of vinegar and tiny bit of Dawn or Joy soap, just a drop or so to help lubricate the vinegar but not enough to be sudsy. I used those microfiber cloths in sets of two, I used one for wet cleaning and the other one for drying. As the wet one became filthy, I would rinse it out and then immerse in the vinegar/soap solution again and clean some more. Once it became too dirty, I replaced it with another clean microfiber rag for more cleaning. The drying microfiber rag stayed pretty clean as it was just removing any traces of wetness left behind from all the vinegar cleaning.
It was amazing the dirt and crud I found, in my case the odor seemed to be strongest under the forward over cab mattress. Vinegar kills numerous types of mold as well as cleans just about anything without damage. I let the cushions and mattress I pulled out get a good airing in bright sunshine. Sunshine is nature’s cleaning agent for many odors. I opened every window and vent in the motorhome while cleaning. Fresh air does magic. I also replaced the AC filter with a cleanable kind. I used a vacuum cleaner to do all the cushions and upholstery, plus vacuumed the mattresses. Amazing the amount of dust and crud that clogged up the HEPA filter in the vac. I had to keep cleaning the HEPA filter over and over, because once it clogs, the vac loses power.
All that massive cleaning seemed to get rid of the odors and whatever stink was under the forward mattress, never came back after I cleaned it up, aired out the mattress and so on.
My carpet was worn out old too. No amount of vacuuming would clean it and I decided it wasn’t worth the cost of steam cleaning due to its age and severe fading and trampling, so I eventually ripped it all out in favor of vinyl tiles. If your carpet is worth keeping, I would rent a cleaner or have it professionally steam cleaned.
Oh and then the obvious… go pay to camp somewhere with a sewer hookup and give your gray and black tanks several good fresh water rinses. Also the hot water tank can hold some nasty odors, vinegar can clean those out too, fill up your fresh water tank with about 10 gallons of plain white vinegar, then turn on the hot water until your hot tank is full of vinegar. Let it sit for at least a day or more, then flush out the entire system with fresh water. You might be shocked was comes out of your plumbing.
Pour boiling water down sink and shower drains, this will loosen up soap scum and get it moving along.
If your camper is going to sit closed up, or if you are living in it with windows closed up due to heat or AC, then invest in some Ozium gel containers. Then open at the top and that stuff eliminates odors without masking it with perfume. Also a germ killer will freshen up an RV and keep it fresh, but it does require electricity.
I usually get my Ozium at Amazon but some car places carry it too.
http://amzn.to/1N1cG4h
My Germ Killer came from Amazon, not sure which other stores stock them. The UV-C bulb has to be replaced every year to keep it working effectively. Mine is about 5 years old and I’ve been through about 6 bulbs keeping it running.
http://amzn.to/1N1dhDc
I am fulltiming with a dog and cooking often, so when I have to keep things closed up for heat or AC, these two products keep my RV smelling like…nothing! No odors, no perfume scents. But anytime the weather is agreeable, I open all the windows to air the place out, nothing like fresh air and sunshine to keep a place smelling fresh. 
At that point, I turn the germ killer off and close up the top of the Ozium.
Which on another note… my friend came to visit in their old car and we went out to eat and their car stunk to high heaven of something awful that we couldn’t find. He said that odor had been there awhile. So I gave him a brand new Ozium gel and we put it inside the car with all the windows closed up. The next day, whatever the stink was, it was gone and he says it never came back.
Source: http://www.rvforum.net/SMF_forum/index.php/topic,88406.msg796504.html#msg796504
