Heaters in PUPs work well. Most have plenty of BTUs to throw at the cold and will keep you warm in 40-degree temperatures (and lower). However, a lot of people don't like the noise. They are pretty loud. The heat gets into the bunk ends but not as well as one would like unless you use a fan. There are some 12vdc fans available for this purpose.
"Since a pup has no insulation, ..." That touches on one of the main issues. There is the issue of heat generation and then the issue of heat retention. Get a set of Pop-Up Gizmos. These go on the top of the bunk ends outside. They provide a thermal barrier that holds heat in when it's cold and reflects the sun to keep the interior cooler on hot, sunny days. Next, buy some Reflectix, which is another thermal barrier product (Lowes or Home Depot in the construction materials section). Cut pieces that are the size and shape of all of your windows. Insert them between the screen and the interior flap. This will cover about 90% of your sidewalls and make a HUGE difference in interior warmth. Finally, insulate the floor by putting down carpet or a bunch of overlapping throw rugs. If you do these things your PUP will stay warmer and your furnace will run a bit less.
As for furnace operation, a standard PUP furnace consumes about one pound of propane for every full hour of operation. So, as an example, let's say the furnace ran 20 minutes per hour all night (night = 8 hours). That would mean you ran 2.67 hours so you used about 2.67 pounds of propane. The most common size of propane cylinder on PUPs is the 20-pounder.
You didn't ask about electric capacity but the furnace fan will consume power and so will any fan you use internally to move the air into the bunk ends so be sure you have a fully-charged battery. You can get through a weekend this way but if you stay out much longer you will need a means of recharging the battery (or batteries -- I run two wired in parallel to double amp hour availability).