Is it safe to transport a slide in without tieing it down if Im only going 40 miles?

That depends... are you going to keep your speed at 10 mph or lower? Or, what are your options if it slides off the back of your truck into the lane of traffic... and someone runs into it?

I think your insurance agent would have a lot to say to you, starting with... your carelessness voided your insurance for this occasion!!!

Think it through before you make a decision... decide on the side of safety, even though none of the above scenarios are likely to occur.

Without going to the expense of a dedicated tie down system, you could still tie it down using rope and/or ratcheting cargo straps (with a definite preference for the cargo straps). Rope would work if you use a "trucker" knot that allows for getting the rope very tight. You'll have to put some thought into how to tie it down, making sure the camper can't lift up or slide out the rear. Also make sure that the straps can't get cut by sharp edges. I personally wouldn't leave my driveway without the camper being tied down.