Walmart Toilet Chemical vs. RV Dealer Toilet Chemical

There are a couple possibilities other than the brand chemical used. Since you noticed it quite soon after moving to new trailer they are suspect.

Are you camped on a seasonal site with either of the dump-valves left open? A big no-no since it allows fumes to get into RV.

The other case might be that the holding-tank vent which is supposed to stay in the upper part of holding tank may have slipped farther down in the tank into the liquid. Vapor in air, inside tank, can not be drawn outside.