Ive seen those electric fireplaces before, they work fairly good but require full hookups. I used that insulated foil bubble wrap and covered our windows on the inside, even the vent doors in the back, that helped alot with heat retention. Unfortunately nothing keeps the deep cold from coming thru the walls, they are just too thin. I found that once you spend alot of time in the hauler with it cold outside you get acclimated to it and dont need it so warm inside your rig. Frozen water pipes will happen regardless of insulation, its just a fact of life in a RV.
TIP: I used tubular foam insulation to insulate the gap around our ramp, that helped alot also.
