Sounds like your best bet would be a fireproof box bolted or welded to the frame in an out of the way (inside) location. These are fairly small (10 x 12 x 6 at a guess), cheap ($20-$40 dollars) but a bit heavy (say 20-30 pounds). Some have built in locks and others have holes to put your own padlock through. This wouldn't be best for keeping valuables, but should deter casual thieves and any RV disasters (except flooding, of course). And if you seal the papers in plastic bags, they would survive flooding too.
BTW, factory installed safes are generally not that 'safe'. I've heard of several cases where they were pryed out of the floor of high end motorhomes in short order.