Given the constrained space under your Cargo Mate 7.5x16 trailer frame rails—approximately 13.75 inches fore-aft, 10 inches in height, and about 65 inches in length—the best approach is to select RV fresh, gray, and black water tanks that do not exceed 60 inches in length and 10 inches in height. Elkhart RV Parts offers a curated selection of tanks within these dimensions that can fit your setup without interfering with the suspension spring hanger or frame rails. You can filter options by size and price using their Tank Finder tool at elkhartrvparts.com/pages/tank-finder. Maintaining a 2-inch air gap from the floor is recommended, and minor modifications such as adjusting the mounting height from 6 to 8 inches or increasing clearance off the cross beam flange can provide additional volume if feasible. This will help you find affordable and compatible tanks suitable for your solo trips with your dogs.