Agree with Papa. If possible don't keep them onboard or in the tow vehicle. If you MUST leave it in the tow vehicle, keep receipts from day 1 purchases (bait, food, hotel etc) so you can reasonably prove you fished two days.
Some of the fisherman friendly lodging locations have both cleaning tables and freezer space available for guys to use. Leave it where ya lodge and swing back there to pick up after day 2.
Also...some cleaners can keep it refrigerated overnight for late day 2 pickup.
so far you have got good advice. we come up to erie for 2 weeks in july. we keep each days catch back at the campgrounds. you never want to have more than one days catch on the water with you. and you need something to prove you were there for each days catch. its best if you can mark each days catch with a magic marker and put the date caught and who caught them.
Separate each persons catch daily and bag and tag them. If you have your limit do not fish for the other persons limit. Put your line and pole away. You never know when the warden is having a bad day and you are next to be cited.