IMO, you really need to seal it - the deterrents work like this: I'm a mouse. I want to build a nest, and I'm going to build it at the best possible place I can find (able to be warm, safe from predators, etc...). If this place happens to be smelly (peppermint) or filled with irritating noises (ultrasonic repellers), that makes it "less good", but if it's still the best place I can find, I'm still going to build my nest there.
I use the smelly stuff and ultrasonic devices in my van, and they help a lot but don't work 100%. I still end up having to replace the cabin air filter every spring (they really like nesting in that). Sealing up my trailer completely has worked 100% for four winters now.
Personally, I'd remove the underbelly panels and seal everything up. My trailer doesn't have those panels, which made it somewhat easier for me. You can also get MOST of the holes from inside the trailer. There were only a couple (gas line under the furnace, for instance) that I couldn't get from inside the trailer.
