Lots of RVs have heated floors. If you walk up the path where the heating vent runs under, the floor is usually warm.
While you certainly can install heated floors there are some issues:
- They aren't particularly efficient with much of the heat going down and being blown away (ducted heat loses a bit but most comes out into the interior space).
- Electric heat is generally pretty limited. 1500w heater is roughly 5000btu. Your average RV furnace is 20-40,000btu. Even with a 50amp rig, you are going to be limited to maybe 15-20,000btu but 50amp rigs tend to be larger and have the larger furnaces.
- Liquid would be time consuming and expensive to install. Then you would have to develop a propane powered boiler system to feed it (electric would still face the btu problem)
Reality is most RVs get little or no cold weather use, so there isn't much market push to improve the heating systems.