Had hot water yesterday. Temp overnight was -3 and now no hot water at any tap. The high got up to 26 for a high and still no hot water in the RV.
Haven't been using the floor furnace; so thought that might be the culprit; but switched off the small poratbles and let the furnace (both front and rear) run for a couple of hours; still nothing?

Any ideas?

I am almost sure that you have a frozen water pipe leading to the water reater or the pipe coming from the heater is frozen. You will just have to warm up the area till the pipes thaw.