Rather than putting a tarp over the top can you build a stand-alone roof with a foot or two space between the RV and roof? That would let some air circulate around the RV and you'll also be able to use your roof vents. A tarp would also cover your plumbing vent and air conditioner.
I agree with others that you should see about getting propane delivery for a big tank. It's the cheapest way to go. For heat you might want to consider a catalytic heater made especially for RVs. They are extremely economical on propane and give a nice steady warm heat. RV furnaces turn off and on as their running and a lot of the heat goes right outside through the vent. We used a propane heater all the time during the day but never at night. We like sleeping in a colder room. Turn it on in the morning and within 10 min. your RV will be toasty. We loved it. Here's a sample:
http://www.campingworld.com/olympian-wave-8-catalytic-safety-heaterFor winter buy the silver foiled insulating material for your windows - some or all of them & especially cover the windows at night.
Good luck!