I didn't see anyone mention using the front of the Tug. After shopping front receivers and new bike racks that would fit on the front of the Tug without a receiver, I found that an old 'trunk' rack that was taking up space in the garage worked just fine.
It's a basic "D" frame rack with extenders for the bikes, with straps intended to connect to the trunk frame. Instead, the straps connect to the front of the engine compartment. The top of the "D" rests on my grill (which is backed by engine compartment framing) and the bottom rests on the frame plate infront of the radiator. The bottom strap connects to the bumper, just like it would on the trunk.
Dirt, rain and security are obvious problems, but the ease of use - being able to leave the bikes in place when we make camp and having them available where ever the Tug goes (daytrips into town, excursions to trailheads, etc.) - overruled these concerns. My cable lock is long enough to secure all wheels and wrap around the Tug's frame (front snatch hook). One issue is vertical clearance - if I had bought the $250 mondo trunk bike rack at our local bike shop, instead of using the old one on hand, I would have more adjustability and could have raised the bikes several inches. I have 'rubbed' the tires crossing drainages.
I tired storing the bikes in the RV, for the reasons Donna listed (dry, clean and out of sight). Scratched my woodwork, limited access to my goodies, and made roadside dining difficult.
Another solution would have been to install a front receiver and use our other receiver-mount bike rack on the front. But I would have lost my front snatch hooks (which give me peace of mind 4-wheeling on the beach), and been out few hundred bucks. But I have heard from others that a front receiver has great utility - 'pushing' the trailer into tight spaces (backyard) instead of backing in.