We've got a 2011 Starcraft 235fb, which is a twin to the Jayco 22 and 23 fb. One reason we bought it new was the 90 gallon FW tank capacity (two 45 gallon underbelly tanks) hidden under the underbelly coroplast covering.
While we really like the large tank capacity, I've had to do a few mods to make things usable or safer.
First, a few years ago, I did the overflow shutoff mod recommended by others on this forum. I was losing nearly 1/2 of my FW just on the way to the camping site. It worked great, just must remember to open valves while filling tanks, so they can vent. Here's the link to that thread: http://www.jaycoowners.com/forums/sh...ter+tank+pukes
Then at the end of last camping season, I noticed water dripping from the coroplast, so I pulled down one side of the underbelly cover and found a loose hose clamp. No big deal...BUT I also saw that my fresh water tanks were sagging several inches in between the 3 steel straps that held each tank. Looked very unsafe. Even when tanks were empty, the sag did not correct itself. This spring when I got the trailer out of storage, I decided to fix the problem. I added 2 more steel straps 1/4" x 1.5" x 36" to each tank.
Menards has exact same steel straps as the Jayco installed ones for about $9 each ($36 total). http://www.menards.com/main/tools-ha...289-c-9215.htm
I pushed the tanks back into shape and added the straps. Right where the tanks were previously unsupported and bulging, there are two designed indentations in the tank of about 1.5 " deep x 3" wide x about 23" long. I believe these indentations are supposed to give some rigidity to the tanks in an area where they are unsupported, (although it obviously wasn't very effective). To provide support for this area above my newly installed straps, I cut and dadoed some treated 2x4's and fit them in the indentations.
Anyway, all is now secure and tanks have no bulge when fully loaded. I wish I took some before and after photos, but all is now buttoned up under the belly cover, so I can't even take an after photo. Oh, well...I was in a hurry to get it done.
I know some people will say the tanks are supposed to bulge, but there is no way that the extent of the bulging was normal or safe, in my opinion. We frequently travel with full FW tanks, so it was important for me to feel good about having the tanks well secured.
For those with underbelly covers, I recommend taking a peek under the cover at least once a year. I would not have known of the bulge without checking...and maybe not until it was too late.