Generic 16,000 btu furnace = $600 + tax + install (built into RV cost)
+
Generic 6 Gallon gas water heater = $318 + install
$918 + installation (built into the cost of an RV)
What is the efficiency of the 2 above (look at any of the brands; I don't want to brand bash here)? What is the electrical draw from any conventional RV furnace? Compare this with the German-made Truma (does both water heating & furnacing in 1 very compact unit)...the difference is around $400 ~ $500. For us, it is a total landslide in favor of the truma. Mind you, when we had our camper manufactured, we cancelled both the furnace and water heater (so we start at an advantage: we would not have to scrap existing units).
Ideally, It would make sense, of course, to have a Truma installed at your RV manufacturer's facility, when you order your camper new...