Hi Deb,
The foam topper, that may be more for being able to sleep.... The bed Sunline put in is better then the normal RV camper bed. More springs, better quality, but it is still a camper bed. While I like hard to sleep on, I could not sleep on the straight bed. Cindy needs mush and for sure she could not either. We now have a 3" high quality topper and both sleep well. Some have also used a big thick feather type topper in place of the foam.
And some love to sleep on the rock as it is....LOL
The sheet thing, as said just tuck in the excess. However the looseness bugs Cindy big time. So she puts on these elastic straps across the 4 corners and then tucks them under. (they are sort of like a garter strap deal, do not know what else to call them nor does she LOL...) Then the fitted sheet actually fits tight. I asked where do you buy them? Well we have had them a really long time.... She said look in the sheet section.
Good luck.
John