Unfortunately the laws and regulations concerning propane containers are very confusing and often conflicting . There is an important distinction , most RV's have DOT CYLINDERS , some MH's have permanently mounted ASME TANKS . ASME tanks are much heavier . If the horizontal cylinders have a saddle at the back so it can stand vertically it usually means it has to be filled in the vertical and used in the horizontal . If you can find a size horizontal ASME tank to fit your opening it still might be a problem being on a slide . My old Avion has 2 original 20# horizontal ASME tanks that fit side by side . I have not tried to have them filled in place . I have had trouble having them filled some places because they look different and do not have OPD valves . They are exempt but fighting ignorance is a loosing battle . I spent 2 hours in my state Fire Marshals office trying to get definitive answers to no avail . I suggest a call to Manchester Tank ,they may be able to provide better information . Good luck
