Most all RVs are built with those little (sometimes big) gaps and spaces in out of the way spots. The concern in manufacturing is the "in your face" areas and those get sealed pretty well. It is up to the buyer to figure out how far they want to go with the "sealing up" of their rigs.
I go through the trailer and seal up all the gaps and even take care of spaces that could become a dirt trap or "housing" for bugs and critters. My 210 had large gap between the floor and the back wall inside the vanity. Large enough that one of my cats jumped down in there and couldn't get out. I thought I was going to have to take the trailer apart to extricate the cat, but, managed to get her out without doing anything drastic. It was where the plumbing came up for the bathroom sink. I took plywood, cut arcs for the pipes (two pieces of plywood to interlock around the pipes) and fitted it between the floor and the back wall. I figured if a cat could fit down there so could other things. That sort of thing is a "user" determination, but, IMHO, any "sealing" or closing up of gaps and holes to the outside will keep the trailer warmer in the winter, cooler in the summer, more pest free and quieter all the time.
My recommendation? Seal 'er up! Can't hurt (most anything you seal with can be "undone" if need be) and most likely will be beneficial in the long run.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)