Meh. We have single pane windows in a used coach that saved us 10's of thousands of dollars when we bought it six years ago. We have camped in the 100's °F but more often in 30's °F. We just do weekend camping trips. We don't have any plans to live in our MH.
Are double pane windows essential? Hardly. The high number number of air leaks and 1" thick foam insulation (or less) in many locations around and under the MH contribute much more heat and cooling loss than the windows.
Double pane windows would be nice but I am happy because I didn't pay for them.
Your demands may be different.
