Mine is simple and brutish.
It can pre-heat the engine, heat the coach interior, and supplement engine heat if trapped in idling traffic for extended periods in winter weather.
A large DBW 300 plumbed in-line into the forward direction piping, in the middle bay, towards the big stock heat exchanger and the stock defroster.
A bypass switch is installed to allow the stock coach blowers to engage without the engine running. A source of battery charging is required or the big fans will shortly deplete the batteries.
A Trace 4024 will consume 10amps of 120 AC to make the 24 volt power for the big Webasto and fans in this configuration.
From an efficiency standpoint, it is a poor performer: Draws in outside air, heats the coach engine block, burns relatively a lot of fuel, BUT it can take the interior to 80 degrees in the middle of a Canadian winter, if you please.
And will raise the engine temp from stone cold to above 100 degrees in 20 minutes with outside temps hovering around -20.
Lots of ways to do a more efficient job, depending on how you will use the coach.
Cost benefit calculation, how many days of use versus the costs of changing things, versus acquiring different parts...
If parts are on hand, you can burn a lot of fuel before you break even on spending more money on other parts.
Generally speaking, if many days of heating a year are anticipated, a coolant boiler (Webasto) type system is likely more expensive to operate than a propane furnace. It all depends on what you get the coolant boiler to do for you.
An article of interest: http://busnut.com/forum/index.php?action=articles;sa=view;article=43
The glory of bus converting: Whatever you do will be right for you!