We have a corner bed. I wouldn't want to make it every week, but once a season isn't that big of deal in trade for the length/floor plan we wanted. Here's what I do using a full size flannel sheet set:
-Fully make bed at beginning of season (or some frequency that works for you). This involves putting a mattress cover over the memory foam topper+mattress and then a flannel fitted sheet over the mattress cover. It takes some wrestling, and bending up of the trapped mattress corners.
-Use the flannel top sheet as your bottom sheet. Just spread it on the bed and tuck it between the mattress and the wall (or under wherever you can reach - this is much easier than getting the fitted sheet on). Between the flannel-on-flannel and being tucked in, the sheet functions just like a normal bottom sheet; we don't have problems with it coming off.
-A spare flat sheet we had at home is now our top sheet, then add blankets as usual.
After each trip, it's quick and easy to strip and remake the two flat sheets and we leave the fitted sheet on the mattress until the end of the season. Haven't tried this system with non-flannel sheets, so I don't know if the tucked-in sheet would slip off in that case or not, but maybe worth a try even if you're not a flannel sheet person.
There are more ideas in this thread, too:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27192680/gotomsg/27229668.cfm#27229668