Here's what we've done:
Bought a couple of used/decent hybrid mountain bikes (less "pain" if they get stolen or damaged)
Received a dual bike cover for Christmas
We have a 2" hitch factory-mounted on the back of the trailer
Have a Bell bike rack (more about that in a moment)
I lock the bikes with a cable lock, to the hitch - when all else fails, the bikes should just scrape along behind the trailer rather than create a problem for other drivers....
We also have a hitch-mounted spare tire: I tied a rope to that, and looped it around the covered bikes to take out some of the bounce.
Back to the Bell rack: it had a plastic 2" to 1-1/4" adapter; and that appeared to be wearing down, allowing an uncomfortable amount of up/down play in the rack. Since we were going to be visiting my bro-in-law, I ordered a metal 2"-to-1-1/4" reducer and had it shipped to his house - THAT made a huge difference in the stability of the bike rack, plus gave about 4' extra, to get past the spare tire.
You might be OK without a bike cover - but our neighbor (from Ohio) at one RV Park, spent about a week getting his bikes to work again after they were bathed in grime and road salt on his trip - and just froze up with rust.... when we got home, the entire backside of our 5er was crusted with salt and grime, but the bikes looked like new.