My Roadtrek is well insulated from noise - something has to be very loud outside to hear it inside. We stay at a campground next to an amusement park and we can only hear the roller coaster when we go outside. As far as temperature inside, if my Roadtrek sits in the sun on an August day the temperature can go up inside - no windows open, no vent open, no fan or A/C running - up to 110 dF. A lot of that heat is coming though the windows and if we covered the front windows, particularly the skylights in the roof, with Reflectix when we left it parked we would cut that heat considerably. We have been in the Roadtrek in the cold months and it is comfortable inside, but I noticed on our last trip when the temp outside at night was in the lower 40s that when I opened the bath door I could feel cold coming from the wall. At the time we had the heat pump on and the temperature inside overall was around 70 dF. The seals on the van doors are stock from Chevy or Sprinter and they seal well.
If you concerned about when you are inside, with the A/C in the summer and the heat pump or furnace in the cold months, it is comfortable inside. You also have the ceiling fan in the summer that just open when leaving the van will take the heat out and if on, will draw the heat out quickly. (When that fan is on a window has to be open for air to move.)