Bob, the best product for your situation would be air bags. They bolt in between your frame rail and rear springs of your truck. Inflate them to +/- 50 PSI when you put the camper on, and you should have a much more stable truck. Then, when you take the camper off, deflate them to about 5 PSI (you don't want to go lower or it may do damage to the bags) and the truck will ride just like it always did before when empty.
Air bag kits can cost as little or as much as you want. You can get just a simple rear bag kit, or you could get the kit plus fancy compressors and pressure controls. I consider an on-board compressor and simple control to be essential because you'll want to deflate the bags if you drive the truck around solo at all when you're out somewhere. Unless you have a compressor that you carry with you, this can really make it much easier. In most cases, you should be able to add a compressor/control afterward if you'd like, if you would like to test out the system first.
You can purchase front air bags, but I doubt that you'd need them for the camper. The beauty of a rear system and control too is that you can adjust your pressures while you drive, if you feel you need a little more on one side. Sunline truck campers are fairly well balanced, but an air bag system will help you adjust each side individually if you feel it needs more on one side.
Jon