Lucite storm windows on the inside creating a dead air space is the way to go. Many ways to hold them in place such as thumb turn clips from Home Depot or Lowes or Menards etc. Cheap and very easy to work with as any simple saber saw will cut it (dont use a real coarse tooth blade as it can crack it, about 2 teeth of the blade in contact at all times when cutting) and the edges easily sand smooth and straight.
A few seconds to put them on or take off as wanted. I only use the thick Lucite etc about .200 thick as it stays flat and seals so much better. I've even put 20% or 40% solar film on the Lucite to filter the sun's rays.
I've done 5 RV's this way on all windows for winter at around Zero F use and it works great! Stops cold air and sweating. Also help with A/C as it lessens the sun's heat and also insulates.
A superb CC LB 4X4, GM HD Diesel, airbags, Rancho's, lots more
Lance Legend TC 11' 4", loaded including 3400 PP generator and my deluxe 2' X 7' rear porch
29 ft Carriage Carri-lite 5'er - a specially built gem
A like new '07 Sunline Solaris 26' TT