We have a diesel SMB with a PH top, bought it used without a furnace. With shore power, a little electric space heater was PLENTY. Used this guy, which we got from amazon for something like $17. http://www.laskoproducts.com/heaters/model_5420.html
. We got hot, even though it was 15 degrees out and we used it on the low power setting. Had it under the PH bed pointed across the lower bed, and there was a pretty decent temp differential between lower and upper bed. Upper bed 15 degrees cooler.
We wanted heat when no shore power was available, so we recently had an espar D2 heater installed. It pulls fuel from the tank, so no need for a propane system, and also did a great job.
Putting reflectix around the PH will certainly help, and will probably reduce the temp differential. viewtopic.php?p=18697
We also got an external windshield cover from Camping World on the recommendation of Greggde, which I'm sure helps as well.
Short version recommendation: If you're ok with no heat without shore power, cheapest serviceable solution is a little electric space heater. If you want heat while dry-camping, go with an espar airtronic if you get a diesel van, or a propane system / furnace if you get a gasser.
Note: some have been happy with a Mr. Buddy heater or a Coleman cat. We have a crowded layout (4 captains' chairs) and little kids, and did not find a safe place to put it when running. We also didn't like the resulting aroma.