There is a huge difference in off road/boondocking heat and full hookup campground heat.
When I had my Casita trailer I looked hard at something like what RoyBraddy is talking about. That was a few years ago and at that time I didn't find anything like what he came up with, namely the AQUA-HOT unit to simplify things. I was looking at car heater cores and mounting options. Never did move on that project for a few reasons, namely I was chicken in that there were few if any installs like it and well, I like tried and true.
Now, with the larger trailer that has its tanks underneath with some exposed pipes, well, I just can't see it working for me in cold weather. (I consider cold below freezing each night).
When hooked up, I love using an electric heater to warm the trailer. By far the easiest and most cost effective.
When boondocking today in cold we don't use the water systems. Instead keep a jug of pink stuff by the toilet for flushing and use water bottles for everything else. When I say cold I'd say expected temps below 20F. Here in NM, a 20F at night will equal a 40 or 50 in the day so not as big a deal. Those of you in cold climates have a whole 'nother set of issues
To the OP's post - I'm sure it has been done, but I question the value of a retrofit. I say that not knowing what you have for a camper - mine has essentially no insulation below the floor. Doing ti might be a chance to fix that