If you use eBay you don't have to auction it. You can do a classified ad the same as Trading Post or Gumtree and still get the same eBay exposure.
We recently purchased a 2nd hand Jayco Eagle OB and spent the 8 weeks prior trawling all the online classifieds.
Ebay, Trading Post, Gumtree, Caravan and Camping Sales, plus dealer websites.
In all honesty I shied away from eBay auctions as it usually meant tight timeframes to inspect (not a chance in hell I'm bidding on something of that value without seeing it in the flesh first), strict buying conditions or some owners seemed a bit prickly to deal with when asked questions like "can I have the vin no. to do a background check" "is any finance owing on it" "can I have your phone no. to arrange an inspection". The seller of an OB Hawk seemed happy to answer the tough questions like "what colour are the curtains" and "is the annex in 'as new' condition", but not my questions... maybe it's me
We ended up finding ours on Trading Post and that was the only place the seller listed it. You would sometimes see a double up on Gumtree (I think it's free to list) or Ebay, but the bulk of campers (Jayco's anyway) being sold were on Trading Post and Caravan and Camping Sales.
Given 95% of buyers probably look at all of the above sites, I wouldn't bother doing multiple listings unless they're free.