I take it you want to navigate 'around' the small roads as in you want to avoid them. There's a camper Tomtom but they're expensive, like £350 expensive. I just use Google Maps to make sure we stay on the main routes.
I have the tomtom camper version, and yes it seems expensive but you can input the weights and size of your MH and it avoids, in most cases, the narrow roads and low weight bridges.
garmin also do one but i have no experience of the product.
my tomtom has saved me a couple of times from getting stuck in narrow roads and at least one with a low bridge.
Google maps will not do this unless you look at the route using street view
shop around for the best price, Currys are doing the tom tom trucker for £299
I have used my Garmin camper 760 LMT-D for 2 years now and it does exactly what it says on the can, remarkable piece of kit, input your size and it will avoid routes that you do not want, and it is loaded with campsites and camper stops everywhere in Europe, please note that I still own a tom tom truck version which I carry along just in case and that too was( still is) exceedingly good at locating poi's that I up loaded on it, sometime I use it just to keep it alive if you know what I mean lol