Have you considered cats?
Check to make sure the hitch "A-frame" box tubing has been closed off behind the hitch ball-socket; stainless steel woven kitchen cleaning pads with some caulk to glue them in place and some spray foam to seal off the voids and keep the critters from pushing their way through it works well...
Then its a matter of seeking out any gap 1/4-inch or larger and physically blocking them as you find them.
However - mice can leap faster than the eye can resolve when they are motivated, leaving the door open while loading items (especially when the weather is turning) has led mice into many many homes...