You should be able to get a helping hand to hold back the piping to re-install the valve assemblies. Did you remove the adjoining piping as well? If so you want to be carefully to spread out the pressure applied to the tank walls, so as to not puncture them when spreading tanks apart. Use some pieces of 1/2" plywood, say 6" square on the walls and a pry bar or 2x4 for leverage. Good luck and keep us posted.
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