We live out in the county in what can only be described as a packrat heaven. Little devils are worthy opponents. And their cousins the gray squirrels.
Look your genset over for holes 1/2" or larger. Plug them or screen them with 1/4" hardware cloth. I believe your access cover has several large air vents.
Do the same with the rest of your rig, too. Use 00 steel wool to block water and electric pass throughs. We do not rig our waste hose unless we are actually dumping.
We have tried all the scents, oils, strobe lights, sound emitters, etc. Unimpressed with all. Don't think you can prevent, only deter.
We circle our rig with 2400 lumen/lm rope lights. Couple of times a week, we will set out a dozen rat traps. 50 or 60 feet away from the rig. Once a week we fire the old girl up and take her out for about an hour. Keeps the rodents from getting too comfortable.
Who says retirement is boring?
Fair Winds and Following Seas