Our two cents: There's nothing forcing a 5kw to run on full blast -- they have a range from roughly 1kw to the 5kw depending on the setting. The 5kw is just the theoretical max.I've read that the 5K heaters are a little to much. Is that true? We'd never camp in below freezing weather.
but between a 5kw and 2kw as your max -- Depends on a lot of factors. 1) How much insulation? 2) How warm is considered too warm? 3) How long for it to warm up is too long? 4) What is the coldest possible outside temp ever expected? 5) If a 2kw was ordered -- would it actually be 2kw or a "de-tuned" 5kw?
For our setup, we used a 5kw and love it. 1) Our insulation is typical but not excessive as we wanted as much interior space as possible. 2) We figured we can always turn down a 5kw -- but a 2kw on max has nothing beyond...3) We don't want to run a heater for hours to keep the entire van warm...so when we do fire it up, we want HEAT! 4) We camp sometimes at 10,000 feet with well below zero...and have plans to see the Northern lights -- 5) A true 2kw is at least as much cost as a 5kw...but not that much smaller.
Hope that helps