Okay, so I know that this is an old post. But, I just recently saw it and that it was recently commented on. So, I figured that I would go ahead and contribute to the thread....
Fist, let me say that the very idea of it being against the law to transport firearms in a camper sounds ridiculous to me, mainly because I simply don't see the point in such an asinine law. I don't know what state you are in but your states law writers must be flamboyantly stupid and proud of it. How can they even pretend to justify such a law?
The above rant having been said in consideration of your dilemma regarding the "no gun in the camper" law, I think that I would probably consider a lockable truck tool box, hopefully one made of steel. I would probably go ahead and lock bolt it to the bed and line the inside with padding. You could even go so far as to add custom slide shelving to hold smaller item.
Though the lockable tool box idea would probably not work very well with a camper shell in place. So, a modified version of the draw you provide a picture of might be necessary. My concern for this would be it interfering with spare tire placement... which is where a lot of folks tend to keep their spare tire. Also, how susceptible to damage would the drawer be, in the event that you went off road?
Also, something you might want to do is look up the law that states that you can not keep a firearm in your camper. Just because a police officer thinks it is against the law does not make it so. In my experience, many police officers don't have enough knowledge of the law to interpret it properly, they rarely have law degrees... that is why we have courts. In fact, we faced a similar "enforcement of misinterpretation of the law" incident in my home town. For years, people where being charged and fined, and in some cases put in jail because of a particular firearm crime, which did not actually exist. The city got pretty beat up over it and faced several law suits, dating back some 20+ years.
When I went to Arizona, from Arkansas, I called every State Police HQ that I would be passing through, to check on the legality of keeping a firearm in the car with me. They all told me that as long as the pistol was cased and unloaded, that I would be fine... also, I had to keep the ammunition in my glove box, not in the case with the pistol. How accurate their interpretation of the law was, I don't know... but, I wrote down the ST HQ officer that gave me the information just in case there was an issue.