We towed a '05 Airstream Classic 30 (31.5' overall/10,000 GVWR) for 12 yrs with a '06 GMC Yukon XL 2500 4x4 8.1l motor, and never had a parking issue. Most of our stays were in National Forests, NPs and State Parks.
Our experience was that most specified length sites referred to the pad surface length. Just using a bit of overhang at the rear nearly always got us comfortably in our site with no Park management Issues.
Same deal with our two previous Airstreams, although they were not as long. If your tow vehicle and you are up to it, I would go large. Even with just two people and two dogs, 2 dalmatians then 2 Corgis, you'll be glad on days you're shut in due to weather!
'17 Thor Tuscany 44MT
'18 Colorado Z71 Duramax
'63 Morgan +4, 18' enclosed hauler