ejs, again welcome to iRV2.com. We are glad to have you join us here and look forward to reading of your adventures and experiences. I am sorry I am not going to be much help in this area, as I am not a chassis mechanic. You may want to check with the chassis manufacturer to be sure that the tank is vented with the cap in place. I thought some tanks were under slight pressure as per design but like I said I am not a chassis guy. Good luck and let us know the outcome and fix.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7