heat wrap should do the trick and they sell it at lowes and home depot or other home improvement stores.
Your best option is to move teh camper to a warmer environment and let it thaw naturally. You don't want to thaw it too quickly and damage the tank. You should look into camper and RV covers. They will help protect your camper from the elements.
I agree with Jessica. Having an RV cover to cover up the RV could help a bit. Also, moving it to a warmer area would also be a quick and easy fix for the holding tank. Is it a hurry? You don't want to rush it and damage the water holding tank. Check out Makarios RV to get the cover. I got mine from Makarios and they offered free shipping. A good enough deal for me!!
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them