After spending a couple of hours under the trailer gazing up and sealing every possible crack, rift, gap and hole maybe use science to erase their presence?
Mouse are basically incontinent - they leave piddle dribbles as a territory thing EVERYWHERE they walk, skip, mosey, camp... and the organics and salts in the urine will glow brightly under UV light... So use UV light to find their pathways and entrance/egress places.
- get a good low wavelength UV 'black light', the low wavelength part is getting it past where human eyes no longer sees it, and go on a mouse trail hunt while in complete darkness. A cheap light has so much 'purple' light it can make fine details wash out.
Yes there are AA-Alkaline battery versions commonly used for freaking yourself out looking in Hotel Bathrooms and finding puppy piddle, and please don't wander into your kitchen accidentally unless you have a weekend to clean - yes the AA powered one will work but for not much more money some lab grade can be had and that cuts down on those 'shades of grey' maybes. Having a cheap aluminum foil reflector on it to keep light from splashing back into the viewers eyes is worth the 90-seconds it will take to masking-tape on in place.
A mouse with a mission can easily sweep in with people entering a building, when they get their mojo on they easily leap faster than crickets and can just vanish in an eyeblink... and don't remove ALL the towels and TP, better the enemy you know than the ones in the walls & other odd hidden spaces?
The days are short and the night is long and the stars go tumbling by.. . ~Airstream~