A 7' width trailer will not handle the RT and FLHT-bikes. Spyders are 5' width and those with bags are 4' plus at the rear, so side-by-side placement is not an option. If you prefer a 7' width, then a 16' length plus a v-nose will allow end-to-end positioning.
Pulling behind an RV allows an 8' width with no real penalty for wind resistance; I pulled an 8' wide trailer behind a van for a while and it was a real chore due to the frontal area. Height is a similar no-penalty concern behind the RV; pulling behind a van or pickup it is a concern for wind resistance and I prefer a lower ceiling height (6' or less). I would recommend a tandem axle over a single axle, given the same load capacity, for bikes or boats: balance, smoothness, handling of road potholes, etc. The tradeoff is an increase in difficulty in tight maneuvering by hand such as in the garage or driveway.
Wayne