HS is probably right to replace with bigger. That said at least one OP here, Grillmeister, has a second tank on his Class C, connected to original with what I'll call a Flow Pipe. From what I understand GM's two tanks fill and empty as one.
But anybody trying to help would need to know Make, Model, Year of your Chassis.
I know for a fact the RV we have now (Signature) doesn't have much, if any, room left under it. Between storage bays, OEM gas tank, Holding Tanks, LPG tank and GenSet, its belly is full.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB