Lenders take a lot of different things into consideration. There’s the basics like income and DTI, but there is also your unsecured debt to income ratio (the ratio of total unsecured debt vs your annual income), net disposable income (amount of money you have left each month after all reported debts are paid. Time at your job, time within your job industry, length of time at your address, whether you rent or own your home. Also, lenders typically prefer your DTI be below 40% WITH new debts figured in. So if OP is already at 40%, he will be above that amount once the RV loan is figured in.

ETA: mortgage payments are viewed the same as any other installment payment. Student loans can sometimes be an exception, but are typically figured at 1% of the balance for monthly payment. The only debt that typically holds almost zero weight is medical debt.