It is true the overall length of the two are different, so a 40' 5th wheel may equivocate to roughly a 35' travel trailer. When you hear a 5th wheel is easier to tow than a travel trailer it is not so much the length differential, rather the difference in the geometry and mechanical differences of the hitches. Where the difference in lenth comes to play is getting into a site. So for example in your case where you have a 37' trailer, you can indeed go to a longer 5th wheel to occupy the same size sites you now do. In trying decide what size unit to get there are many factors to consider and one should not assume that bigger is always better. It is important to consider factors such as will you be travelling frequently from place to place, if so a smaller unit may be a better fit. On the other hand will you be primarily stationary, in which case a bigger unit if so desired would be fine. You also need to look at what are your preferences for campsites ie. National Parks, State Parks or Boondocking where in many cases over 35' you are likely not going to be able to get a space. Or are you going to be staying predominately at RV parks where many have larger spaces. But even in commercial RV parks, larger sites may be limited and or the streets leading into the site may be so tight and narrow, making it difficult to get into the site. IMHO a good approach is to look at the smallest unit you can feel comfortable in, then make the necessary adjustments from there. Having said this if you are happy with your current space and if you cannot see yourself in a smaller unit, you can indeed go to a 40' or so 5er.