Try to stay on good terms with your dealer but remain firm. As PROFESSIONALS they has every opportunity to inspect your old trailer before the deal was done. You have a long business relationship with this dealer that they should want to continue.
Sometimes you need to let people vent before they really have considered what the real problem & solution is. If there are no sign of mice and no smell then I really don't see what the problem is. It would be like if the fridge stopped working, was fixed and now works. Problem solved.
Stay strong & calm - read your purchase agreement carefully to make sure there are no suprises.