Typically you won’t have space or the weight capacity to have a fireproof safe in your RV. When it comes to protecting documents against the elements, a good alternative is a fireproof/waterproof document bag. They’re lightweight, easy to hide, and can protect essential documents in the event of a fire or flood or natural disaster. Thieves, on the other hand, that’s a different topic.
Pro Tip: Since we travel full time, we keep our passports and other necessary paperwork in one of these bags. We do not keep anything of financial value (cash or banking data) and go “out of our way” to hide it.