River Otters usually have their kits about this time of year and will often move on after the kits are born. Attempts to drive the mother away could result in babies dying under your trailer so you may want to be sure that's not the situation that would only add to the problem.
Kits are not ready to leave the den until they are about 3-4 months old.
I am offering you some options below to help with the smell you have currently.
To start with you'll need to somehow rake under the trailer to remove whatever you can. (wear a mask so you're not overcome) If you don't remove the source of the smell then it will return . Otters leave behind things like fish parts,etc.and Otters tend to have certain places that are used solely for toilets.This could be a part of the smell.
Using regular household bleach /water with a spray bottle would be tedious and a carefully used pressure washer might help after you've raked out as much as you can. It's difficult to do I know and you may need to ask someone to help .
There is a product sold specifically to help kill the odor and it's here:
http://www.wildlifecontrolproducts.com/cgi-bin/ez-catalog/cat_display.cgi?account=X355889;search=2;search=Animal%20repellents%2C%20Animal%20removal;limit=category;v=2.0 (Scroll down to "One Shot Deodorizer")
I think I would also use a sonic pest repellent year round or coyote urine and you'll find that at the website above also to keep the rascals away. Using a fence that measures 3 x 3 (7.6 cm x 7.6 cm) or smaller wire mesh before you replace the siding can help prevent them from returning. Hog wire type fencing can be used but otters can enter through it by making the bottom wire larger, so you'd have to check that type more often.
The office listed below is for The Conservation Commission who may have help to offer you concerning your problem.
Vancouver Island Regional Office
#3 - 1200 Princess Royal Avenue
Nanaimo, BC V9S 3Z7