Back to the OP's question - Definitely want to block both rear tires on each side as needed. Don't just do one on a side. Reason is it has two tires for a reason - to better support the weight. Even if just for a day, two, or four, you would be stressing the one tire being blocked by carrying all the weight.
'13 Forest River Sunseeker 3100
'04 Fleetwood Sedona / Reese Mini 350 / Prodigy / BAL
'94 Coleman Cedar