Kudos to Skyline, I found some of their owners manuals are still online, even one dating back to 1968! I couldn't find any 'rules' but the owners manual for one Rampage and it talks about partial water in the tanks.
- If possible, totally fill or empty the water tank to minimize “sloshing,” which may affect towing stability.
- If possible, empty holding tanks to minimize “sloshing,” which may affect towing stability.
I assume the OP bought this used, which now makes me wonder if it might have been a rental. Back when I bought our 2005 Raptor I looked at a Rampage, I sure don't recall any rules when I went through the unit back then.