[JRTJH]

In the Atwood line, here's the frame hole measurements:

6 GALLON W H 16˝ 12.5˝ 18.5˝ Weight 25 lbs
10 GALLON W H 16˝ 15.5˝ 21.8˝ Weight 32 lbs


In the Suburban line:
MODEL GAL BTU WIDTH HEIGHT LENGTH WEIGHT
SW6DE 6 12,000 12-11/16” 12-11/16” 19-3/16” 37
SW10DE 10 12,000 16-7/32” 16-7/32” 20-1/2” 50
SW12DE 12 12,000 16-7/32” 16-7/32” 22-1/4” 52
SW16DE 16 12,000 16-7/32” 16-7/32” 27” 53

As you can see, the frame opening size is the same for the 10, 12 and 16 gallon Suburban heaters, but the 6 gallon size is significantly smaller.

Remember that the framework in your trailer is aluminum, so you'll need to not only cut the frame, but reweld (or screw/bolt) the new frame back into the opening. With 50+ pounds, you'll need a substantial frame around the new water heater to support it during towing.

The newer "high recovery" water heaters, when used on electric and propane do a pretty good job of keeping up with the demand for hot water. With a recovery rate of 16 gal/hr, that's 22 gallons of hot water counting the 6 gallons in the tank. Mixed 50/50, that's 44 gallons of "shower temp water", mixed 25/75, you'll still get over 35 gallons of "shower temp water"...