When covering our motorhome it has been my experience that the covers need about six more straps that run under the belly from side to side.
It's pretty windy on the hill where we live so I like to make sure that the sides of the cover are nicely tucked under so the wind has less to catch.
If you would like to give your cover a little cleaner look and keep the edges tucked under, here is a little trick that I do that doesn't ruin the cover.
Go to any hardware or lumber store and depending on the size of your camper get anywhere from 6 to 10 soft clamps, and a 100 foot of clothesline rope.
Drill a 1/4 or 3/8 inch hole through one handle of the clamp for the rope to fit through.
Just tie one end of the rope through a clamp, fold up a little section of the cover so the clamp has something to grab, and clamp it on. Measure out the length of rope you need to make it to the other side, cut it, tie to another clamp, pull out the slack in the cover, and clamp it on.
That's it! You can do that anywhere you feel the cover has a little extra slop in it.
when you're all done it will keep your edges tucked neatly under the camper.
Edit- I should add that the clamps come in many sizes and strengths so the better the clamp, the better it will hold.