More than likely, EVEN if you do find a camper that has a mattress your size, it won't be comfortable. Almost every mattress factory installed is junk. Mattresses are usually the VERY first think RV owners change when getting a new camper. (of any make, model, design, or class).
We got lucky with our Outback. It actually came with a decent mattress. We never had to swap it out. Previous 2 trailers we did.
About the mattress? Chances are you will swap it out anyway. So, if mattress is the #1 draw back, then take a tape measure with you when looking at various campers and measure to see if a standard size (length) mattress will fit. (check with slides IN!!!!). Even if the bed frame is a bit small, you can always add an extension support for the end of the mattress that might hang over. So, just because it's a short mattress, does not mean you can't swap it out. Just make sure the camper has room for a long one with everything buttoned up and all relevant slides retracted.
About "tubs"? IF you find a trailer with a bath tub, it will be so small, it will be virtually unusable. Your camper will probably have a 6 gallon water heater, which will not even be enough water to adequately heat, even a small bathtub, with enough water for even a small child to take a comfortable bath.
If your baby is a "baby", you are best to just get a child-size bath tub that can be stuck in a closet when not in use. Also, if the camper has a shower, you can always put a cover over the shower drain, fill the basin with an inch of water, and put the baby in the water on the basin floor.
6 gallons of hot water will not go very far... Tubs are useless in a camper. Teach the toddler to take a shower, or just simply give him/her a cat bath. Or better yet, hose him down outside.
Bunkhouse models are popular floor plans. But really ... kids will sleep anywhere and DO adjust to anything! Get a camper that is comfortable for you and your spouse FIRST! Kids second! Kids are along for the ride! You are the one paying the bill. Especially if your child is a toddler, he/she won't know anything different. Get what works for YOU first! Chances are very great that in 2 - 4 years you will be getting a different camper. As the kids grow, you're ideas will change too. So you really don't need to make this purchase a life-drama event or stress over it. It's not a life-long commitment. You will eventually change for a different camper. It just happens!