As Dave pointed out about the trailer brakes. The camper is electric brakes and your tow vehicle is hydraulic. Water can effect each vehicle a very differently. If you are pushing down on the pedal you can tell from the response of the truck if the brake pads on the truck are slipping/wet or dried off to make a controlled slow down. This can cause a little concern on what will happen when the electrical pulse activates the trailer brakes. In normal conditions you want the trailer to very slightly engage first before the truck in order to have a straight line controlled slow down. If you are in a heavy rain and feel that the trailer will act in the same way as if drive 'dry', the many bad things can happen and they will happen fast. If the trailer brakes 'dry' faster than the truck brakes you can be thrown around from the front end of the truck as the trailer violently pulls you 'back'. You may end up driving in the ditch or be over in the on coming lane. If you have too little braking from the wet trailer brakes and the truck is slowing down faster than the trailer, the back of the truck will be hit hard with a driving force that will push your back end around and could force you in the ditch or on coming traffic. This will cause a jack knifing effect. Whether to have too much or to little trailer brake, lean to the side of too little with the idea of only depending on doing the stop with the power of the truck. This will cause you to drive slower and keep a longer distance between you and the vehicle in front of you. If you are driving from point A to point B and have an idea of how long it will take to get there, add 25% to that time frame.
Also, in driving in the rain, be aware that it is possible for a positive/negative air flow to happen in your camper and rain could be sucked in and get a few things wet. On our older camper a side window became harder to close completely, we also had a front window with a full cover shield. In heavy rains, water was always being sucked up under that 'shield' and through the louvered window and onto our bed. It did not bother me to much as it was on the wife's side