T@B Electrical and Rain

I carry an XXXL Ziplock bag with my electrical supplies. If it is storming, I slide the surge protector through a hole I've already cut in the bottom of the bag, connect it to the electric pedestal and do my electrical checks.  Then I turn the breaker off, connect the power cable to the side of the T@B, connect it to the surge suppressor that is in the bag, close the zipper as much as I can, use those flexible orange ties to keep the bag tight to the cable and surge suppressor,  tuck it under the lid as much as I can, and turn the breaker on. This should keep all of the connections dry. It works for me. 

Short Answer:
The 30A power cord to the T@B twist locks about a quarter inch and then there's a ring that should be gently tightened (easy to strip) that will seal elements from the trailer side. I've never had an issue at any campground in severe weather where power cord and surge protector combo lay had to lay on the ground to get soaked in a puddle and never had the male end get "wet" when properly plugged into the 30A receptacle on the campground's box - even with surge protector. If ever I end up in a unique area where the electric box is so low to the ground that the male end (+surge) sits on the ground, I'd do exactly what Verna has described for extra protection.

Long Answer and Stories (aka tl;dr version):
I've had small trailers since the early 90s. The ones I possessed had simple wiring, 12V plugs and 12 lights and maybe a couple 110 v outlets (not even outdoor ones).  The only "delicate" electronics would be the converter itself in those trailers. For the 13 yrs I had the popup, I ran mostly on battery, but never had power issues or blown fuses when plugged in because of weather.  The Reverse TD had a perfect area for a large solar panel and I was 90% on battery when I was out with it. The T@B is the first trailer I've owned that had any phantom draws or delicate electronics (or even a frig!!). This is the first time I've used a surge protector.

I haven't taken my T@B to the varied unique places many people have taken theirs and not encountered severe weather while at a campground (heavy rain doesn't count). That said, in the past year and a half, my T@B has seen hail, snow and some intense rainy weather recently while parked beside my house. One storm with hail fried my entire FOIS outdoor data box a week ago. I had the T@B plugged in (as well as my motorcycle tender to the bike). I don't have a garage. The electrical cord feeding both T@B (with surge protector) and motorcycle hangs off the ground. It's sheltered under a motorcycle cover (with bike). That's it. So far, I've had no issues. The motorcycle tender is plugged in and connected to the parked bike all the time. The T@B is plugged in sporadically as I've modified my bin, battery location and solar set up. I also periodically top off the T@B battery after having had the main fuse pulled for a while.

Many campground electric boxes can actually close shut (without portable surge protector on). Good quality larger rigs have built-in surge protectors. Progressive Industries has one that is highly recommended, but around $300. I'd have it professionally installed. Improperly tightened connectors inside the box can come loose due to the bouncing that a small, light trailer experiences on the road (let alone off-road). If you do the job yourself, check the tightness of the connections periodically.

Always step back when opening up campground boxes at the beginning of the season. Wasps and spiders like the protection too (Imported fireants love electrical boxes. They are attracted to the electrical current - something that was always annoying to have to deal with when the air conditioner unit would suddenly stop working in mid July in TX). 

I know others will give their experiences and you will find several different answers. Do what you feel most comfortable with for you and gives you peace of mind. I worry more about improperly wired campgrounds and low voltage "spikes" than weather when I have the 30A cord plugged into a receptacle.

Best to you and yours as you navigate the systems of your new T@B! It's a fun (sometimes frustrating) learning experience. This forum has been so helpful. In the end, all of us want to relax and camp no matter how we define that concept.