Use the max pressure printed on the tire side wall. That is the pressure which gives them the rated weight capacity.
In my case with the "D" rated radials, it's 65 PSI.
If you have 55 PSI tires, they may be load range "C." "D" tires are required to cover the full GVWR of a camper. I'd check the sidewalls again for a weight rating and compare it to the real weight of the camper to make sure the tires are not overweight.
The only time I've had a blow out was when a shop put "C" rated tires on a camper. It is amazing how ignorant a tire shop can be about camper tires.
2012 Rockwood A122S
2008 Toyota Tundra Double Cab 4X4
Former owner of a 2002 Coleman Niagara GTE