I answered a similar thread a while back, I used 12v hotwires in a ply floor for a customer the only downside is they required about 15 amps to give any heat off the floor. So ideally the engine needed to be running to keep the leisure battery topped up to a reasonable level.
Not sure about amps and the likes but I'm guessing its similar situation to heated seats. If I put the seats on for more than an hour the battery is pretty much dead. Also like the seats (we use like an elec blanket) I'm guessing it won't really heat the whole van very well.
Trying to run any sort of heating off a 12V leisure battery simply doesn't make sense unless you want to run the engine all the time. If you've done any camping in anything other than summer conditions you'll find that 500W is the minimum size of heating that will give any sort of result (and even that won't be comfortably warm in a van in winter).
If you have a pop-top you'll need much more than that - perhaps 1kW for winter use.
Simple maths with a 500W minumum gives a current draw at 12V of 40A, which will kill a top quality, fully charged leisure battery in less than 3 hours (in theory - in practice it will kill it a bit quicker, and it will probably damage it in the process).
Running 240V panels while on EHU is of course a different matter. 500W only takes 2A, so no big strain on your hook-up and no massive cables needed. However, all of the underfloor heating panels and mats I've looked are meant to be used under tiling or laminate, which distributes the heat and stores it to a certain extent. It's also designed more for background heating, so you'll struggle to get anywhere near enough area in a van floor to install 500W, and the need for insulation under the elements will raise the floor significantly.
I looked at fixed electric heating in my current van, and considered these (wall-mount) panels:
Been thinking of getting some heating in the van for some time now...would only need it when plugged in camp site been looking at plinth heaters these can be got with anything frm 800watt up to 3kw ....the one am looking at can be switched frm 800w or 1.3kw
Any one running a plinth heater or are they a no no T:
i installed two 240v 120w tube heaters in mine with a simple room thermostat for when im on mains hook up they will run off my 500w inverter but as the previous posts state it is a high current draw and only beneficial for a few hrs. tube heaters are allot safer than other convection units and allot more compact. the thermostat also means you can set the van to a comfortable 22 degrees.
Another great tip/thing i did was install universal heated seat pads in to the bed cushions. i now have a toasty non damp bed to get in too and on the low setting they last all the way through the night.T: