Tow, yes! Put in water?? BEATS ME!
I have a 1996 Coachmen 300QB.
I tow my 3100 pound Jeep behind the MH all the time. You can't even tell it is there. Gas mileage is unchanged.
You will slow down going up steep mountain roads. But not a lot more than with just the coach.
I have no idea whether you'd want to put the boat in the water with the motorhome. It will depend on tongue length, etc.
I wouldn't do it.
The 300QB has a long overhang behind the rear wheels. Distance from ground to the Frame is 20 inches. From the bottom of the frame to the coach floor is about another 20 inches, maybe a couple less.
Everything back there on the bottom is theoretically waterproof.
The 'theoretical' part is why I wouldn't do it.
A 300QB's walls are made by making a sandwich of two sheets of 1/4 inch lauan 4x8 sheets with a 1" sheet of blue Dow extruded polystyrene sheet between them. The polystyrene is EXACTLY the same thing you can find at Lowe's or any home improvement store.
Here's how it is protected from water. The outside is covered by a very thin layer of fiberglass (brand name is 'filon') glued with what appears to be contact cement. The inside is covered by some vinyl wall paper. The undercarriage portion of the inside of the sandwich also has some synthetic cloth glued to it ... but loosely.
The edges of the sandwich - including at the bottom where putting a boat in the water would probably get submerged - are covered by an aluminum trim piece, but not water-sealed in any way.
So, submerging the bottom edge of the wall will let water get into the end grain of the lauan sheet. (lauan is a Philippine wood - think plywood and you'll be close enough). The end grain will wick the moisture up into the wood. The wood layers will begin to rot and separate. The separation will cause the 'sandwich' to come apart.
Not good. This is why motorhomes 'delaminate'. It is all about moisture.
In other words, it is not just THIS model motorhome - but ANY motorhome - that I'd want to avoid putting in the water to launch a boat.
The 300QB you are looking at will probably tow your boat very easily. Mine would.
But you 'probably' wouldn't want to put it in the water with the MH unless you could be sure not to submerge the bottom edge of the back wall, or unless you tore off all the fabric down there, removed the aluminum and plastic trim, and went crazy sealing everything with some West System or another brand of epoxy first...
We're enjoying our Coachmen. And even with the heavy 'Maintenance Projects' involved with such an older MH ... it is still far cheaper than any comparable MH I could find ... and is very livable, and highly upgradeable, inside.