I did some heater work on my DEV bus.. my rear heater on that Carpenter bus had a regular blade fan in it as opposed to a blower.. it was noisy and had a somewhat low BTU rating.. (I took it out and gave it to a friend who is restoring a similar vimtage carpenter that had a leaky one).. and I replaced my rear unit with a Jegs unit. my reasoning was I wanted more BTU out of the rear heater since when iom mobile-officing I sit in the middle of the bus.. my Mid-heater A/C is a power hog, the Jegs heater is quiet and ductable.. you can duct it however you like, and it has a mucbh better heat output than the old single-fan underseat heater did.. with my 12kw coolant heater, that single heater will heat my whole work area.. occasionally I turn on my stepwell heater for a little extra..
thye only thing about the Jegs heater is the lines are 5/8 so they do work best in a parallel flow system where slowing the flow at the rear is actually desired.. however you'll still get decent flow through a series system, it just may lack a little at idle. (slightly cooler air)..
if you want to duct it.. (theres a TON of other things for ducting.. i have used these kits for ducting my custom A/C and heat installs.. although I like the coyote vednt louvers better these are good ones too..