The Coleman LED battery powered Lateran should work well. We have 2 of them in the older florescent type. Ours Runs on 4 D batteries. We also bought one of the new single LED ones that points down and reflects up. That thing is so bright it is too bright for inside. This type if for a big area.
The 6 watt solar panel, it will not be effective enough to do much with charging your battery. Skip this item.
I do not have any experience with the Mr Buddy so cannot help on that one other than caution.
A few tips. You will need to turn into a power miser and you can be surprised at just how long you can go.
As was said, if it is going down below 50F, vent the camper, crack open a roof vent or window at night. The condensation from breathing over night is actually unbelievable. If you keep the camper sealed up tight trying to keep heat in, the walls can end up soaked. This gets worse when the camper is heated.
Power, remember to turn off the TV antenna booster. That thing will suck a lot of power left on doing nothing.
If you have not converted inside lights to LED, heads up the standard 921 light bulb that comes in the Sunlines draws a lot of power. 1.4 amps worth. Use one sparingly as needed. Do not be having 4 or 5 lights on for long periods of time with the standard bulbs. In time, buy a LED plug in replacements in the common use spots. You can run 5 to 6 LEDs for the power 1 of those 921 bulbs draw. Use the lanterns to conserve power until you get the LED some day.
The fridge on LP does not use much, it is OK but make sure if yours has a condensation heat strip between the freezer and fridge compartment it is turned off. That heat strip sucks a lot of power.
I myself pull the fuse on the back of the radio so that draw is not there. Since I do not use it camping, it is not loose to me. I do use it when at home working ont he camper but I have power then
The HOT water heater, only run it when you need it.
The furnace, the blower uses a lot of power, use it sparingly if you are on 1 battery.
Make sure the battery is fully charged before going out boondocking.
Keeping an eye on battery state of charge. As was said, you need a digital volt meter ideally with 2 decimal places. One decimal place you get you buy but 2 is better. They do not have to cost a lot either. $10 to $20 buys a good enough meter. Radio Shack use to have a small one but I no longer see it on their site. On line their smallest is $24. A Harbor Freight one will do the job if they have a store near by. A good analog meter (has a needle on it) is also OK just the better ones cost a lot and now a days digital is very cheap and reliable. If you really get into this they make a Volt Minder complete with an alarm on it but that is in the $45 range.
See here, there is a chart on the bottom of this linked page for a unloaded battery on the 12 volt column,
Trojan Battery Company
You really do not want to discharge down below 50% at lot or many times doing this. The batteries last longer if you use that as a low end target. So 12.10 volts is the low end target to drain a battery to before recharge. It is OK to recharge higher than that, just do not drain it down a lot more then the 12.1 as read in a resting state. You may dip below 12.1 when the furnace is running but it will come right back up when the furnace shuts off pending where the volt meter is located.
Before testing the battery voltage you need to have not been on a charger or connected to the truck for a good 6 to 8 hours. Then turn everything off in the camper but the fridge and measure the voltage. You are getting a true'er reading then. You can then figure out how much capacity you are using each day as you watch it drop and then how much you have left. Unless you measure it you really do not know if you have a lot left or very little.
With being a power miser and keeping and eye on things, you can do 3 days OK.
Your GM truck, yes the 7 wire receptacle on the back is live to the truck battery all the time even with the ignition off. Pull the 7 wire plug or it will drain the truck too. Many Fords and Toyota's have a relay in them. Other brands may as well, just not many of the older GM's. Don't know if they ever changed that on the new trucks.
Using the truck as charger, yes it will put a level of charge into the battery but it depends on how low the battery is to how long the truck has to run to charge the battery back up. While better than nothing a 300 plus HP engine running with the truck standing still trying to recharge a heavy drained camper battery 20 some feet way on a small wire is going to take a real long time. Remember a stand alone 12 amp charger can take 24 to 48 hours, to recharge a battery drained down.
Good luck, you will do fine. Caution: This can get addicting!
The boondocking places are often nicer then the power sites.
Let us know how it went.