What kind of marks are you talking about? Black rub marks (from rubber transfer from other boats rub rails, for example) can usualy be scrubbed off with a bit of effort. Some oils penetrate the material and can't really be taken off no matter what you do. Same with a lot of tree saps. Depends on what you have to deal with.
If you do go the solvent route, give it sufficient time to evaporate out of the fabric before covering.
Car brake cleaner ,also gets oily marks off fabric car seats,carpets etc,not sure what it actually is, it used to be trico ethylene before that was banned,it smells like a dry cleaners.
Spray it on a wad of paper roll or a clean rag and give it a rub, it's not as severe as acetone so probably won't clean as well but it won't start to melt the surface either.
