SilverEscape wrote:
I remenered one I found a year or two ago. Livin Lite Camplite 21 BHS. Meets all of your criteria except for the heated tanks. For the enclosed underbelly, it looks like the option is spray on insulation. So you might want to call Livin Lite and see if it what they do would work for you. And ask them about heated tank options, they may be able to offer a suggestion.
https://www.livinlite.com/camplite21bhs-overview.php
This is what we have. It's been towed about 6,000 miles, and we used it a lot last year. It has been an excellent camper. As SilverEscape said, the 21BHS meets all of your criteria except the enclosed underbelly. However, I read on another forum about someone who just ordered a Camplite with an enclosed underbelly so perhaps that is now an option.